add a label flag for the build command

[kneal]

Is your feature request related to a problem? Please describe.

Not related to a problem but looking for additional functionality from the makisu build command.

The Docker CLI and other utilities for building docker images often offer a --label flag to allow you to not only add labels via the label instruction within the Dockerfle but while building the image. This allows the user additional flexibility when adding labels to the image.

Describe the solution you'd like

It would be great if I could add a flag when doing makisu build -t myimage --label foo=bar . that allows me to add new labels that don't already exist within the Dockerfile.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

N/A

Additional context
Add any other context or screenshots about the feature request here.

N/A