feat: allow specifying Kaniko targets for multi-stage dockerfiles

[hamishc]

I've encountered issues with large multi-stage dockerfiles when targeting one particular stage. For example, if we have a devcontainer.json like

{
    "name": "envbuilder",
    "build": {
		"dockerfile": "Dockerfile",
		"args": {
		},
        "target": "devenv"
	},
}

and a Dockerfile as the following

FROM alpine AS runtime-deps

FROM runtime-deps AS devenv

FROM runtime-deps AS long-build-stage
COPY some-build-artifact/ /

then running docker run -it -v /my/repo:/workspaces/empty coder/envbuilder:latest will result in the following error

Failed to build: do build: error building stage: failed to get files used from context: failed to get fileinfo for /workspaces/empty/.devcontainer/some-build-artifact: lstat /workspaces/empty/.devcontainer/some-build-artifact: no such file or directory
Falling back to the default image...
error: do build: error building stage: failed to get files used from context: failed to get fileinfo for /workspaces/empty/.devcontainer/some-build-artifact: lstat /workspaces/empty/.devcontainer/some-build-artifact: no such file or directory: no fallback image has been specified
error: running command "envbuilder": do build: error building stage: failed to get files used from context: failed to get fileinfo for /workspaces/empty/.devcontainer/some-build-artifact: lstat /workspaces/empty/.devcontainer/some-build-artifact: no such file or directory:
    github.com/coder/envbuilder.run.func4
        /home/hamish/code/envbuilder/envbuilder.go:271
  - no fallback image has been specified

This is because we don't specify the build target in KanikoOptions. Specifying this option can also substantially decrease build times, especially when combined with the SkipUnusedStages option - which can allow for large unused build stages to be avoided entirely.