Only pushing if merge on main

[ineiti]

With this PR, the docker images are built, but only pushed when the action is run on the 'main' branch.

Thank you for opening a pull request with this project, please also:

• add a brief description of your changes here
• assign the PR to yourself, or to the person(s) working on it
• start in draft mode and in progress pipeline in the project (if applicable)
• if applicable, add this PR to its related issue by one of the special keywords Closing keywords
• once it's ready
  • put it in the Review or Ready4Merge pipeline in the project (if applicable)
  • remove draft
  • assign a reviewer

[PascalinDe]

LGTM