Non-git directories should error on tags

This happened because of an edge case with a git binary version on a self-hosted runner.

When using tags of branch or sha on a directory that isn't a git repository, you get a complete error as an image name, e.g; eu.gcr.io/project/image:0.1-fatal: not a git repository (or any parent up to mount point /github) Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).-fatal: not a git repository (or any parent up to mount point /github) Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

This is an invalid image name, and kubernetes had some issues with this.

Expected Behaviour

Should fatal or exit back to the user (with a non zero exit code)

Current Behaviour

Creates invalid image names

Possible Solution

Exit with a non-zero exit code (so that a pipeline can exit too, in this case, the runner had checked the code out as a zip instead of git without any notice)

Steps to Reproduce (for bugs)

1. Run faas-cli build --parallel 5 --tag=sha -f faas-stack.yml on a non-git directory

Context

Runner had changed the checkout method without notification

Start by reproducing faas-cli build --parallel 5 --tag=sha -f faas-stack.yml from a non-git directory and trace the build tag handling. Done means the command exits non-zero for this condition and does not create an image name containing the Git error; add or update coverage if the repository has tests for this command.