Support selecting between multiple Dockerfiles via parameter

[dmildh-absci]

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.

[dmildh-absci]

Ah looks like there is a ENVBUILDER_DEVCONTAINER_DIR var. I spoke too soon