Jan Pazdziora

2015-03-06

When building an image from Dockerfile using

docker build .

the FROM image:tag specifies the base image to be used. So it is possible to define not just rhel, centos, or fedora, but be more specific: rhel7.0, centos:6.6, or fedora:20.

The subsequent

RUN yum install -y ...

or

RUN yum upgrade -y ...

commands will use whatever repositories are configured for yum. For Fedoras, every version has its own repos but how about operating systems that use major.minor versions? Let us test:

docker run rhel7.1 rpm -q yum
yum-3.4.3-125.el7.noarch

FROM rhel7.1
RUN yum upgrade -y yum

docker build -t rhel7.1-yum .
[...]
docker run rhel7.1-yum rpm -q yum
yum-3.4.3-125.el7.noarch

Now with RHEL 7.0:

docker run rhel7.0 rpm -q yum
yum-3.4.3-118.el7.noarch

FROM rhel7.0
RUN yum upgrade -y yum

docker build -t rhel7.0-yum .
[...]
docker run rhel7.0-yum rpm -q yum
yum-3.4.3-125.el7.noarch

The package got upgraded to the version from 7.1 even if we said FROM rhel7.0. That however only means that the base image we start with is 7.0, yum will use the 7Server directory just like with 7.1.

To make sure yum is fixated to a particular minor version, let us set releasever:

FROM rhel7.0
RUN echo 7.0 > /etc/yum/vars/releasever
RUN yum upgrade -y yum

docker build -t rhel7.0-yum .
[...]
docker run rhel7.0-yum rpm -q yum
yum-3.4.3-118.el7.noarch

And it works not just for installing/upgrading the yum package:

FROM rhel7.0
RUN yum install -y httpd

docker build -t rhel7.0-httpd .
[...]
docker run rhel7.0-httpd rpm -q httpd
httpd-2.4.6-31.el7.x86_64

FROM rhel7.1
RUN yum install -y httpd

docker build -t rhel7.1-httpd .
[...]
docker run rhel7.1-httpd rpm -q httpd
httpd-2.4.6-31.el7.x86_64

FROM rhel7.0
RUN echo 7.0 > /etc/yum/vars/releasever
RUN yum install -y httpd

docker build -t rhel7.0-httpd .
[...]
docker run rhel7.0-httpd rpm -q httpd
httpd-2.4.6-19.el7_0.x86_64

This is useful especially for testing when you want to make sure you get packages from the old version and not the latest minor release.