Support additional build arguments (not --build-arg)

[Roemer]

Currently, build has an object args which can be used to pass --build-arg to the docker build command.
But it is often needed, to also pass other command line arguments, like --network or --add-host. This is currently not possible in an easy way.

I see two possibilities:

1. Rename args to vars or variables as they just set build-time variables and re-use args for the additional cli-arguments. This would be more in-line with runArgs but introduces a heavy breaking change.
2. Add a new property cliArgs or buildArgs (would then be build.buildArgs) to the build section which are then passed to docker build. This would just be an extension and not break any existing dev-container definitions.

This would solve the issue microsoft/vscode-remote-release#3545 which already has almost 100 upvotes.

[sliekens]

I would also like to mention it's not currently possible to pass ANY build args when using a combination of image and features, even though this will for sure do a docker build .

{
  "image": "mcr.microsoft.com/devcontainers/base",
  "features": {
    "ghcr.io/devcontainers/features/node:1": "lts"
  },
  "build": { // ERROR: property build is not allowed
    "cacheFrom": "ghcr.io/owner/repo/devcontainer:latest"
  }
}

As you can see, property build is not allowed when using image although features would benefit from caching from a pre-built image to speed up builds.

[scalpel4k]

we have the "runArgs" for 'docker run', why can't the same be done for 'docker build' using let's say "buildArgs" ?

[Roemer]

BuildArgs have a special meaning in docker build. They act like placeholders and are used in the Dockerfile with ARG. This is what we already have with build.args.
What we want here is actually cli arguments passed to the docker command. So we should call them build.buildArgs (actually a bit misleading) or better build.cliArgs or build.commandArgs or something like that.

[hakontonne]

Why not call it build.options? That would keep this in line with the official docker documentation that explicitly names them options:
docker build [OPTIONS] PATH | URL | -

https://docs.docker.com/engine/reference/commandline/build/#options

[Roemer]

Yep, that's what my proposal, ehm, proposes: options (#328)

[chrmarti]

This is in progress now: #324 devcontainers/cli#85. Thanks @Roemer !