In the last post we quickly looked at how you can install everything you need to run docker containers on a CentOS 7 host and did bring up the official PostgreSQL image. However there are good reasons not to rely on an existing image: You want to deliver a pre-configured base image which includes everything your application requires and you want to have control over that image. When this image needs to get updated you can update the base image and then re-deploy it to wherever you need to update the base image.

For CentOS there is a script which you can use to build your base image on GitHub. For other distributions you can check the docker documentation. This script basically makes use of the “–installroot” switch of yum which allows to install packages to another location than the default one.

Using this script we can build a CentOS 7 base image. There are three parameters you can use:

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-p ""  The list of packages to install in the container.
                 The default is blank.
-g ""    The groups of packages to install in the container.
                 The default is "Core".
-y      The path to the yum config to install packages from. The
                 default is /etc/yum.conf for Centos/RHEL and /etc/dnf/dnf.conf for Fedora

We’ll use the “-p” switch to install all the packages which are required to build PostgreSQL from source (this depends on the compile options, of course) and some additional packages which are useful for daily work (such as screen):

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[docker@centos7 ~]$ sudo ./mkimage-yum.sh -p "gcc openldap-devel python-devel readline-devel redhat-lsb bison flex perl-ExtUtils-Embed zlib-devel crypto-utils openssl-devel pam-devel libxml2-devel libxslt-devel tcl tcl-devel openssh-clients bzip2 net-tools wget screen ksh unzip" centospg

Once done the new image is available locally:

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[docker@centos7 ~]$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
centospg            7.2.1511            184509483d52        40 seconds ago      510.6 MB
postgres            latest              78e3985acac0        2 days ago          264.7 MB
hello-world         latest              c54a2cc56cbb        5 months ago        1.848 kB

If you wonder how that image made it into docker take a look a the last lines of the script:

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tar --numeric-owner -c -C "$target" . | docker import - $name:$version

What happened is that a tar file of the temporary chroot directory was created:

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[docker@centos7 ~]$ ls -latr /tmp/mkimage-yum.sh.dKHtsq/
total 24
drwxrwxrwt.  2 root root    6 Aug 12  2015 tmp
dr-xr-xr-x.  2 root root    6 Aug 12  2015 sys
drwxr-xr-x.  2 root root    6 Aug 12  2015 srv
drwxr-xr-x.  2 root root    6 Aug 12  2015 run
dr-xr-x---.  2 root root    6 Aug 12  2015 root
dr-xr-xr-x.  2 root root    6 Aug 12  2015 proc
drwxr-xr-x.  2 root root    6 Aug 12  2015 opt
drwxr-xr-x.  2 root root    6 Aug 12  2015 mnt
drwxr-xr-x.  2 root root    6 Aug 12  2015 media
drwxr-xr-x.  2 root root    6 Aug 12  2015 home
drwxr-xr-x.  2 root root 4096 Aug 12  2015 dev
dr-xr-xr-x.  2 root root    6 Aug 12  2015 boot
lrwxrwxrwx.  1 root root    7 Dec 10 15:02 bin -> usr/bin
lrwxrwxrwx.  1 root root    8 Dec 10 15:02 sbin -> usr/sbin
lrwxrwxrwx.  1 root root    9 Dec 10 15:02 lib64 -> usr/lib64
lrwxrwxrwx.  1 root root    7 Dec 10 15:02 lib -> usr/lib
dr-xr-xr-x. 17 root root 4096 Dec 10 15:02 .
drwxr-xr-x. 13 root root 4096 Dec 10 15:02 usr
drwxr-xr-x. 17 root root 4096 Dec 10 15:02 var
drwxr-xr-x. 19 root root 4096 Dec 10 15:02 etc
drwxrwxrwt. 11 root root 4096 Dec 12 11:43 ..

… which then was imported into docker with the “docker import” command.

To test if it really works we can start it and execute bash:

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[docker@centos7 ~]$ docker run -it --rm  centospg:7.2.1511 bash
[root@cf690e9d9476 /]$ cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"
 
CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

Fine, so far so good. Now we can really begin: We have our own CentOS base image where we want our PostgreSQL image to be based on. How then can we create a PostgreSQL image on top of our base image?

We’ll execute the commands necessary first and provide explanations afterwards. So, the first step:

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[docker@centos7 ~]$ sudo yum install -y git
[docker@centos7 ~]$ mkdir centospg
[docker@centos7 ~]$ cd centospg
[docker@centos7 centospg]$ git init
Initialized empty Git repository in /home/docker/centospglatest/.git/
[docker@centos7 centospg]$ git config --global user.email "daniel@abc.def"
[docker@centos7 centospg]$ git config --global user.name "Daniel"

Why do we need git? It is not required to use git at all but you probably would like to have your files which are used to build your containers managed by git so that you can use all advantages of GIT combined with the advantages of docker. It will will make more and more sense as we step through all the commands.

What we need to create now is a so called Dockerfile. This file lists the instructions that Docker will execute to build you image. Lets go:

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[docker@centos7 centospg]$ touch Dockerfile
[docker@centos7 centospg]$ git add Dockerfile
[docker@centos7 centospg]$ git commit -m "initial" Dockerfile
[master (root-commit) ce3727a] initial
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 Dockerfile

Our very basic Dockerfile will look like this:

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# use our CentOS base images as source
FROM centospg:7.2.1511
# set the PostgreSQL we will download
ENV PG_VERSION 9.6.1
# include the PostgreSQL binaries in the PATH
ENV PATH /u01/app/postgres/product/96/db_01/bin:$PATH
# add a postgres group and postgres user
RUN groupadd postgres
RUN useradd -g postgres -m postgres
# prepare the directories
RUN mkdir -p /u01/app/postgres
RUN chown postgres:postgres /u01/app/postgres
# allow sudo for the postgres user
RUN echo 'postgres ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
# download, configure, compile and install PostgreSQL from source
USER postgres
RUN wget https://ftp.postgresql.org/pub/source/v${PG_VERSION}/postgresql-${PG_VERSION}.tar.bz2 -O /var/tmp/postgresql-${PG_VERSION}.tar.bz2
RUN cd /var/tmp; tar -axf /var/tmp/postgresql-${PG_VERSION}.tar.bz2
RUN rm -f /var/tmp/postgresql-${PG_VERSION}.tar.bz2
RUN cd /var/tmp/postgres*; ./configure --prefix=/u01/app/postgres/product/96/db_01
RUN cd /var/tmp/postgres*; make
RUN cd /var/tmp/postgres*; make install
# cleanup
RUN rm -rf /var/tmp/postgres*

Using this Dockerfile we can build our PostgreSQL image:

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[docker@centos7 centospg]$ docker build -t centospg:PG9.6.1 .

The output of this is quite long, here a snippet:

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Sending build context to Docker daemon 45.06 kB
Step 1 : FROM centospg:7.2.1511
 ---> 184509483d52
Step 2 : ENV PG_VERSION 9.6.1
 ---> Running in 054900c7ebe1
 ---> 866815b9f092
Removing intermediate container 054900c7ebe1
Step 3 : ENV PATH /u01/app/postgres/product/96/db_01/bin:$PATH
 ---> Running in 46bcf7667a06
 ---> 94c9adb0402b
Removing intermediate container 46bcf7667a06
Step 4 : RUN groupadd postgres
 ---> Running in 24a7d9b7a1ea
 ---> eb4ff8268e2e
Removing intermediate container 24a7d9b7a1ea
Step 5 : RUN useradd -g postgres -m postgres
 ---> Running in 3e09b556fed8
 ---> acff1dcf2d4c
Removing intermediate container 3e09b556fed8
Step 6 : RUN mkdir -p /u01/app/postgres
 ---> Running in 999a62d075c0
 ---> fa4bdfa74d31
Removing intermediate container 999a62d075c0
Step 7 : RUN chown postgres:postgres /u01/app/postgres
 ---> Running in 37773e484260
 ---> 668c491b534b
Removing intermediate container 37773e484260
Step 8 : RUN echo 'postgres ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
 ---> Running in bb9cbfd20623
 ---> 589959efbda5
Removing intermediate container bb9cbfd20623
Step 9 : USER postgres
 ---> Running in f70b8c70c3fc
 ---> 32d3d3d603d2
Removing intermediate container f70b8c70c3fc
Step 10 : RUN wget https://ftp.postgresql.org/pub/source/v${PG_VERSION}/postgresql-${PG_VERSION}.tar.bz2 -O /var/tmp/postgresql-${PG_VERSION}.tar.bz2
 ---> Running in c5cc11840a15
--2016-12-12 12:43:05--  https://ftp.postgresql.org/pub/source/v9.6.1/postgresql-9.6.1.tar.bz2
Resolving ftp.postgresql.org (ftp.postgresql.org)... 174.143.35.246, 217.196.149.55, 87.238.57.227, ...
Connecting to ftp.postgresql.org (ftp.postgresql.org)|174.143.35.246|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 19260568 (18M) [application/x-bzip]
Saving to: '/var/tmp/postgresql-9.6.1.tar.bz2'
 
     0K .......... .......... .......... .......... ..........  0%  180K 1m44s
    50K .......... .......... .......... .......... ..........  0%  368K 77s
...
 ---> 645cf59717f4
Removing intermediate container c5cc11840a15
Step 11 : RUN cd /var/tmp; tar -axf /var/tmp/postgresql-${PG_VERSION}.tar.bz2
 ---> Running in 6a47968ddeb5
...
# PostgreSQL configure, make, make install
...
PostgreSQL installation complete.
 ---> 7f6b11b357d7
Removing intermediate container 041441816c4d
Step 16 : RUN rm -rf /var/tmp/postgres*
 ---> Running in 480cc2157b9a
 ---> a7b0bf1d1c35
Removing intermediate container 480cc2157b9a
Successfully built a7b0bf1d1c35

Once all of that completed we have a new Docker image:

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[docker@centos7 centospg]$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
centospg            PG9.6.1             a7b0bf1d1c35        45 seconds ago      706.7 MB
centospg            7.2.1511            184509483d52        47 hours ago        510.6 MB
postgres            latest              78e3985acac0        4 days ago          264.7 MB
hello-world         latest              c54a2cc56cbb        5 months ago        1.848 kB

Using this image we can now bring up a container which complies to all our standards we build into the base images:

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[docker@centos7 centospg]$ docker run -it centospg:PG9.6.1 bash
[postgres@7ac7780b0b1b /]$ which initdb
/u01/app/postgres/product/96/db_01/bin/initdb
[postgres@7ac7780b0b1b /]$ initdb --version
initdb (PostgreSQL) 9.6.1

Finally we commit our Dockerfile:

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[docker@centos7 centospg]$ git commit -m "First working version" Dockerfile
[master f0ba897] First working version
 1 file changed, 25 insertions(+)

Something to start with, isn’t it?

Note: The Docker best practices tell you to add only few instructions to a Dockerfile because every instruction creates a new layer. In general you should run only one service per image. This makes it easier to scale you containers.

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by
Daniel Westermann