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