Fix automatic tagging of buildx builds

[glennmatthews]

Closes: #N/A

What's Changed

• Add a new print_command helper function to tasks.py to generalize some logic that was specific to the docker_compose function and provide a common entry point for printing a CLI command before running it.
• Replace hard-coded default tag of networktocode/nautobot-dev-py3.7:local on buildx task to derive appropriate automatic tags for dev and final images based on the specified Python version - I needed the print_command to help me demonstrate that this was working correctly. :-)
• Add print_command in a few other cases.

$ INVOKE_NAUTOBOT_PYTHON_VER=3.9 invoke buildx --target final
Building Nautobot final target with Python 3.9 for linux/amd64...
PYTHON_VER=3.9 \
docker buildx build . \
    --platform linux/amd64 \
    --target final \
    --load -f ./docker/Dockerfile \
    --build-arg PYTHON_VER=3.9 \
    -t networktocode/nautobot-py3.9:local \   <------------<<
    --no-cache

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• n/a Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• n/a Outline Remaining Work, Constraints from Design

[jathanism]