Docker builder should support separate dockerfiles for dev and debug

[briandealwis]

Users expect that skaffold dev and skaffold debug with file-sync will ✨magically✨ rebuild or relaunch their applications built using the Docker builder.

Although we do expose the SKAFFOLD_RUN_MODE as a build argument for docker builds, it is difficult to craft a Dockerfile rules to take advantage of it.

I can see two approaches:

1. Use the specified dockerfile and see if there is a ${dockerfile}.${runMode} (e.g.,Dockerfile.dev or Dockerfile.debug). But it's impossible for the user to override.
2. Allow the dockerfile to be templated so that they can incorporate the SKAFFOLD_RUN_MODE environment variable.
3. Extend the docker: definition to allow specifying separate devDockerfile and debugDockerfile items.

[tejal29]

adding p2 priority, unless @briandealwis you know this is higher priority.

[briandealwis]

p2 sounds good