Fix Docker image updater action

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).
• You have marked this pull request as a draft and added '[WIP]' to the title if needed (if you're not yet ready to merge).

⤴️ Summary

Fix incorrect branch specification in Docker image updater workflow

🌂 Related issues

n/a

🔬 Tests

n/a

[craddm]

LGTM