feat: add dkim signing

• task from #1

🚧 Tasks

• docker container build set to allow_fail

  doesn't yet need to be built as part of ci. In the future when tests are created it will be set to build then.

• All submitted mail is dkim signed, including between users

• add to dockerhub publish, if on master branch, add an additional tagged image latest if dev branch dev tag in addition to versioned tagged image.

• a merge to dev should also push docker dev tag even if no release

• Add .dockerignore for all files not part of the image