Allow to add custom tag for image building in the pull request

[triffer]

Description When building an image for a PR, that tag will always be in the format of PR-${PR_NUMBER}. You can provide a tag to the build, but it will be ignored. It is not obvious to the user that the tag is being ignored in this case. We found out that the PR tag is hardcoded.

Reasons

Since we want to create multiple images with different features for a PR, we need to be able to specify a tag when we build an image for a PR.

Acceptance Criteria

• [ ] Providing a tag to an image build for a PR will use the provided tag.
• [ ] The default tag is preserved when custom tags are provided.

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