Support Build Args during Docker Build

[eedwards-sk]

I seem to be unable to specify build args during the dev container build process.

https://docs.docker.com/engine/reference/commandline/build/#set-build-time-variables---build-arg

I want to perform an npm install during the build process, but this requires providing an environment variable for npm private registry authentication.

a build arg would be the way to accomplish this (an env var)

I would like a way to provide build args during the build process, from file (so e.g. an .env file), e.g. "build_arg_file": "npmjs.env", :

{
	"name": "foo",
	"context": "..",
	"dockerFile": "Dockerfile",
	"build_arg_file": "npmjs.env",    
	"settings": { 
		"terminal.integrated.shell.linux": "/bin/bash"
	},
	"extensions": [
		"dbaeumer.vscode-eslint"
	]
}

[Chuxel]

I believe this is a dupe of #46. Lets continue chatting as needed over there. Would be great if you would upvote as well!