`metal up`: build an image

[rgarcia]

The server should build an image!

Rough outline:

• unpack the targz to a directory,
• run nixpacks build with the --out option so that it outputs a Dockerfile
• run docker build using build cloud https://docs.docker.com/build/cloud/usage/

You can stop short of pushing the image somewhere--I anticipate these steps will be tricky as is, so want to just focus on getting an image built.

[linear[bot]]

MET-20 `metal up`: build an image