feat(manager/gitlabci): add support for registryAliases (#16227)

* feat(manager/gitlabci): add support for CI_REGISTRY

* fix: pass correct property

* chore: adapt tests

* docs; add documentation for the new option

* test: extend test cases

* fix: do the abc properly

* refactor: switch to using registryAliases option

* docs: mention registry variable

* style: remove extra heading

* docs: fix sentence

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

* docs: move documentation to managers

* docs: fix formatting

* refactor: handle default value in function body

* refactor: remove unused function

* refactor: move registry alias handling to dockerfile manager

* style: remove unused import

* refactor: use match groups directly

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* refactor: add dockerfile manager tests

* chore: fix variable name

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>