Support Docker buildArgs

[thernstig]

Currently the Remote Containers support the devcontainer.json runArgs. It would be great to amend it with a buildArgs. This would open up possibilities to stop at specific build stages with --target, or anything else one can imagine when building. --build-arg and --pull are two other great options that come to mind that one would want to use.

https://docs.docker.com/engine/reference/commandline/build/
https://docs.docker.com/develop/develop-images/multistage-build/#stop-at-a-specific-build-stage

[thernstig]

Another issue that could be closed if this is implemented is this one: #46

[egamma]

Duplicate #46

[thernstig]

@egamma #46 specifically only mentions the --target build argument, whereas this issue mentions setting any build arg. So I did not consider it a duplicate when writing this, but as long as #46 is solved with a generic build arg then all good :)

[egamma]

@thernstig the issues are linked so this additional info will not get lost.