Update Docker image instructions

[abitrolly]

Leave only relevant details and mention how to specify Ubuntu version

Addresses #2673 (comment)

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh tests/unit?

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[cjp256]

With or without the suggestions, +1

[codecov-io]

Codecov Report

Merging #3499 (3a1be8c) into master (405c576) will not change coverage.
The diff coverage is n/a.

❗ Current head 3a1be8c differs from pull request most recent head b0d48d3. Consider uploading reports for the commit b0d48d3 to get more accurate results

@@           Coverage Diff           @@
##           master    #3499   +/-   ##
=======================================
  Coverage   90.94%   90.94%           
=======================================
  Files         276      276           
  Lines       18826    18826           
=======================================
  Hits        17121    17121           
  Misses       1705     1705           

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[abitrolly]

Rebased.