Docker selector

[OlegKunitsyn]

Some programs valid for GC 2.2. are not compiling in GC 3.0 or GC 3.1. For now, Docker image is hardcoded to gnucobol:3.1-dev. I would like to change "container": "gnucobol" property to the list of:

• "container": "gnucobol:3.1-dev" by default
• "container": "gnucobol:3.0"
• "container": "gnucobol:2.2"

What do you think about?

[GitMensch]

In general this shouldn't be a list but an URL, possibly with the newest one as default.
Reasoning: people would like to use their own docker containers which have additional tools installed.

It is definitely useful to provide multiple containers as some people have a version to match, and so may want to limit themselves to an earlier version. Thinking of this... What do you think about providing a gnucobol:1.1 container, too? This would also allow #81 to be tackled more easy some later day...

side note: there's work in progress on alpine (actually on adelie, but this will be pushed to alpine afterwards) to have gnucobol available via the system package manager - which will make the container setup more simple, too.

Some programs valid for 3.1 will not be valid for 2.2 - but not the other way around. The only exception are newer reserved words which may need to be undefined via -fnot-reserved=word1,word2.
Please post to the bug list or issue tracker or discussion board for any program that compiled before and now doesn't (actually we provided the 3.1rc1 to catch those issues and fix them before 3.1 final - but that's only possible when we do get reports...)

[GitMensch]

Just to add the reference: fixed with 553a888