How to replace slashes in the branch name

Git branches uses names like `feature/new-feature`, the slash is incompatible with docker image tags so i want to use a regex (or other means) to replace the slash in the $BRANCH_NAME with an underscore. This is possible in bash using the following:

```
BRANCH_NAME=feature/test echo ${BRANCH_NAME/\//_}
```

is there any way to create an env variable with the branch name escaped in cloudbuild.yaml?

Start with the cloudbuild.yaml configuration and the documented handling of the $BRANCH_NAME environment variable. Check whether the requested slash-to-underscore transformation can be represented there, using the bash example and Docker image-tag requirement as the acceptance criteria. Done means the branch-derived value is available as an environment variable without slashes.

Written by the indexing model from the issue text.