update `only:` for release/ branch regex matching. and to avoid running image builds on unnecessary forks

everything

--trace

it is mostly useless, but makes failures harder to find

The artifacts it produces were no longer used since we switched to
mkdocs.

CI: Enable OTP release building for maint/* branches

See merge request pleroma/pleroma!1749

(cherry picked from commit 7c9b023a)

71315800 CI: Enable OTP release building for maint/* branches

Add PLEROMA_BUILD_ENV to override env for the version generator and use it in the CI for building docs

This is needed because if we run with MIX_ENV=dev we will get a "+dev"
suffix to the version in the docs, but if we run with MIX_ENV=prod the
docs build will fail because ex_doc is dev-only dependency.

This reverts commit 54d28737.

Needed so we could push documentation updates to master without
triggering a rebuild

This is useful for the CI builds because they have a detached HEAD or
for emulating a feature that is availible only on a particular branch
(e.g pleroma_ctl downloads CI artifacts only for develop and master)