frizbee doesn't handle docker:// references

[jhrozek]

Describe the issue

Seen when remediating the stacklok/sigstore-the-hard-way repo:

failed to parse action reference ''docker://avtodev/markdown-lint:v1'': invalid action reference: docker://avtodev/markdown-lint:v1'

To Reproduce

Point frizbee to a workflow that references an action with the docker:// prefix.

What version are you using?

No response