Improve dockerfile

[stavros-k]

Additionally:

• Updates the maintainer to point to our mail, and uses the non-deprecated format
• Fixes env style deprecation
• Cleans apt to reduce image size, as it wont work afterwards without a privileged user
• Moves iconik binary installation into a seperate script, so we can also use it in CI to get the version to use in the tag

Now it will create the following tags:
latest and the version of the iconik binary, at the moment of writing, 3.11.3