Do I need separate Dockerfiles for py2 and py3? - docker

Question

Currently I have 2 Dockerfiles, Dockerfile-py2:
FROM python:2.7
# stuff
and Dockerfile-py3:
FROM python:3.4
# stuff
where both instances of # stuff are identical.
I build two docker images using an invoke task:
#task
def docker(ctx):
"""Build docker images.
"""
tag = ctx.run('git log -1 --pretty=%h').stdout.strip()
for pyversion in '23':
name = 'myrepo/myimage{pyversion}'.format(pyversion=pyversion)
image = '{name}:{tag}'.format(name=name, tag=tag)
latest = '{name}:latest'.format(name=name)
ctx.run('docker build -t {image} -f Dockerfile-py{pyversion} .'.format(image=image, pyversion=pyversion))
ctx.run('docker tag {image} {latest}'.format(image=image, latest=latest))
ctx.run('docker push {name}'.format(name=name))
is there any way to prevent the duplication of # stuff so I can't get in a situation where someone edits one file but not the other?

Answer 1

Here is one way using Dockerfile ARGS along with docker build --build-arg:
ARG version
FROM python:${version}
RUN echo "$(python --version)"
# stuff
Now you build for python2.7 like so:
docker build -t myimg/tmp --build-arg version=2.7 .
In the output you will see:
Step 3/3 : RUN echo "$(python --version)"
---> Running in 06e28a29a3d2
Python 2.7.16
And in the same way, for python3.4:
docker build -t myimg/tmp --build-arg version=3.4 .
In the output you will see:
Step 3/3 : RUN echo "$(python --version)"
---> Running in 2283edc1b65d
Python 3.4.10
As you can imagine you can also set default values for ${version} in your dockerfile:
ARG version=3.4
FROM python:${version}
RUN echo "$(python --version)"
# stuff
Now if you just do docker build -t myimg/tmp . you will build for python3.4. But you can still override with the previous two commands.
So to answer your question, No, you don't need two different docker files.