🌱 Update Dockerfile

[Mionsz]

What this PR does / why we need it:

Update Dockerfile, add basic labels and speed-up the build process:

• Dockerfile compliant first line metadata
• use curl instead of git for repository download
• use build-time bash shell with -exo pipefail for error catching
• add proper labeling of resulting image
• further improve image size by reducing layers number using one RUN command.
• missing ENTRYPOINT added
• missing USER added
• other minor improvements

[dtantsur]

Many reasonable changes here, but I'd slightly prefer several pull requests so that we can consider them separately.

A couple of comments (+ requesting input from Riccardo)

[tuminoid]

/ok-to-test

[elfosardo]

thanks for the patch, appreciate the improvements I agree with Dmitry though, this change should be split in multiple PRs