Support selecting between multiple Dockerfiles via parameter

Could envbuilder support multiple dockerfiles in the same repo? For example:

./DockerfileA
./DockerfileB

I haven't found a way yet but a couple options I might include allowing specifying which Dockerfile to use through an environment variable (e.g., ENVBUILDER_DOCKERFILE_PATH), or by supporting variable substitution in devcontainer.json's dockerfile field.

Start by tracing how envbuilder reads the devcontainer.json dockerfile field and selects a Dockerfile, then compare that path with the proposed ENVBUILDER_DOCKERFILE_PATH option. Done means a repository containing multiple Dockerfiles can reliably select the intended one through a documented parameter, with coverage for the selection behavior.

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