Replace static additional members dict with a method for programmatic
modifications.

Resolve "WP-CLI: PHP Warning about missing libraries"

See merge request !21

Resolve "Disable plugins that are not configured for installation"

See merge request !14

This adds a new dependency to the image: jq

Closes #11

This is to acquire the the merge commit for a branch that was merged to
both develop and master seperately. No code changes should result.

Resolve "Incorrect 301 redirect to homepage when 404 would be expected"

See merge request !16

This seems to be the root cause of missing URLs somehow morphing into
redirects.

Resolve "Fix builds post-release of PHP-8"

Closes #9

See merge request !13

Build args in FROM lines NEED to be before any other lines, apparently.

The new config relies on no pre-existing configuration files.

Merge ongoing development to "master"

See merge request !11

Shortcuts ALL the PHP code, not just most of it.