make-container-image: Add FORCE_MW_VERSION support
When environment variable FORCE_MW_VERSION is set when building
mediawiki container images, only the mediawiki version specified in
the variable will be added to the image, and the FORCE_MW_VERSION
environment variable will be set in the image (to be later used by
https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/1053752
and its followup
https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/1056004
which finalized on the variable name across codebases).
Bug: T369115