Add version variable

[casperklein]

Description

This adds a variable to let the container know, which version is running.

Fixes #1946

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[polarathene]

May want to prefix it? (DMS_VERSION)

VERSION is a bit generic of an ENV var name for this imo, but I'm not too fussed.

[georglauterbach]

If you incorporate @polarathene's sugggested changes, feel free to merge without my approval :D

LGTM