Turning a git branch name into a valid docker image tag

When using GitLab CI, you can use a predefined variable CI_COMMIT_REF_SLUG, description:

$CI_COMMIT_REF_NAME lowercased, shortened to 63 bytes, and with everything except 0-9 and a-z replaced with -. No leading / trailing -. Use in URLs, host names and domain names.

Source

Docker tag does not allow most of the special character except -,_,..

A tag name must be valid ASCII and may contain lowercase and uppercase letters, digits, underscores, periods and dashes. A tag name may not start with a period or a dash and may contain a maximum of 128 characters.

docker valid image tags

So you can replace all special character with -. in your Branch name.

docker build . -t image_name:$(git rev-parse --abbrev-ref HEAD | sed 's/[^a-zA-Z0-9]/-/g') 

So the following branch will become

fix/bug#123 -> fix-bug-123  
pr@123 -> pr-123

You can replace - with underscores, periods and dashes

#to use `_`
sed 's/[^a-zA-Z0-9]/_/g'