Support passing `--local context=FILE`

[fenollp]

Description

Today one can only pass directories as local build contexts. I would like to be able to pass a file path as well.

A few workarounds:

• Write a .dockerignore to disk in the right place
• Copy the file to an empty dir and use it as context

Both of these require disk operations however.

If added, this capability should be mentioned at (the great BTW) https://docs.docker.com/build/concepts/context/