Investigate Docker failure during Github Action on push to master

[zarino]

Example:

https://github.com/mysociety/caps/actions/runs/10739837183/job/29786540549#step:3:3556

#15 naming to docker.io/mysociety/caps:latest done
#15 DONE 9.4s
Run docker/login-action@f054a8b539a109f9f41c372932f1ae047eff08c9
Warning: The `save-state` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
Warning: The `save-state` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
Warning: The `save-state` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
Logging into ghcr.io...
Login Succeeded!
Run docker push "$REGISTRY/$IMAGE_NAME:base_$BRANCH"
The push refers to repository [ghcr.io/mysociety/caps]
tag does not exist: ghcr.io/mysociety/caps:base_master
Error: Process completed with exit code 1.```