[DISCUSS] Move docker development into versioned branches

[Chesnay Schepler]

Hello everyone,

Currently, all development in docker-flink occurs on the master branch, 
for all releases at once. The master branch also serves as a publishing 
area for our docker files.

This means that all versions share the same generators, templates (i.e., 
production code that we plugin in some variables like the Flink version) 
and test setup.

So far this worked fine because all versions worked the same, but we are 
now coming to a point where we are adding version-specific features, 
like support for Java 11 (FLINK-16260) and per-job-clusters (FLINK-17164).

By virtue of all files being shared this means that we would have to 
introduce version checks in both the generators, templates and test 
code, so things continue to work for all versions. This is rather 
painful to do, and presents maintenance problems as these conditions 
must of course be updated at some point, and diverging behaviors require 
all versions to be tested on each PR (in contrast to the current state 
where we only test 1 version).

It is also simply different to the process we're using for 
flink/flink-shaded, creating an additional entry barrier.

I propose moving the development of the docker files into dedicated 
branches (dev-master, dev-1.10, dev-1.9). PullRequests will usually be 
opened against dev-master, possibly ported to other versions, and when 
it is time for a release we will update the master branch with the 
latest dockerfiles. Quite similar to how things work in the Flink repo.

The master branch would continue to be our release publishing area to 
stick to docker conventions.

Regards,

Chesnay