docs: improve Docker documentation (#7925)

* fix whitespace in list, let markdown renderer figure out ordered list

* capitalize noun SemVer

* behaviour -> behavior

* use node 14 in examples, get latest SHA256 digest for node 14.15.1

* be consistent in using files in list items

* use docker-compose.yml in item list

* use active voice style for list items

* rewrite sections preservation of version precision and version compatibility

* rewrite section on digest pinning

* fix typo

* rewrite sectoin digest updating

* rewrite section configuring/disabling

* fix typo

* change h5 headings to h3

* Rewrite sections

* use simpler version numbers for examples

* rewrite confusing section

* fix typo

* selfhosted -> self-hosted

* remove double word

* add missing word

* Use myimage instead of node for examples

Co-authored-by: Rhys Arkins <rhys@arkins.net>

* change node -> myimage in another sentence

Co-authored-by: Rhys Arkins <rhys@arkins.net>