ci: only build containers on main branch

[pschmitt]

Description

The containers should only be built when pushing to main.

Motivation and Context

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation change
• [ ] New feature (non-breaking change which adds functionality)

Checklist:

• [x] All new and existing tests passed.
• [x] I have added tests to cover my changes.
• [ ] I have updated the documentation accordingly.

[github-actions[bot]]

:tada: This PR is included in version 3.12.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: