Enhanced Ubuntu-20 Dockerfile readability

[Dunky-Z]

Is your feature request related to a problem? Please describe.
Ubuntu 20's dockerfile looks not tidy enough and has some invalid instructions.

Describe the solution you'd like
Enhanced Ubuntu-20 Dockerfile readability

• Delete some comment symbols such as ##;
• Separate setting locale, update-alternatives and installing package;
• One line per package;
• Add a clean cache step;

Remove some invalid statements

• remove RUN apt-get -y autoremove
• remove RUN apt-get clean

Describe alternatives you've considered
None so far. Comments welcome.

Additional context
I'm working on it, I'll create a PR later.

[osteffenrh]

Merged.