Slugify the branch name used as the docker image tag

[HaozheZhang6]

The Build Docker job tags the image with the raw branch name:

BRANCH_NAME: ${{ github.head_ref || github.ref_name }}
...
docker build -t ghcr.io/cadquery/cadquery-docker:${BRANCH_NAME} -f images/dockerfile .

A docker tag cannot contain a slash, so any PR from a branch named like fix/... or docs/... fails the build before it even starts:

ERROR: failed to build: invalid tag "ghcr.io/cadquery/cadquery-docker:fix/assembly-load-segfault": invalid reference
##[error]Process completed with exit code 1.

That is currently red on #2064, #2065 and #2044.

This replaces the slashes when building the tag. master has none, so the tag pushed to GHCR is unchanged.

This branch has a slash in its own name, so the Build Docker job on this PR is the check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (6d741ee) to head (b915ed7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2066   +/-   ##
=======================================
  Coverage   95.79%   95.79%           
=======================================
  Files          30       30           
  Lines        9435     9435           
  Branches     1405     1405           
=======================================
  Hits         9038     9038           
  Misses        242      242           
  Partials      155      155           

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[adam-urbanczyk]

Thanks @HaozheZhang6, LGTM.

[adam-urbanczyk]

CI fix, merging.