Specify Environment Dockerfile Path In Manifest

[damiantw]

I've found the current convention for Dockerfile paths to be a papercut annoyance for Docker runtime Vapor projects with many environments.

As is, Dockerfiles must be stored in the project root, and chances are they will not be grouped together in an alphabetical directory listing due to the environment name serving as the filename prefix.

In addition, each environment must have its own Dockerfile. Often the Dockerfile contents for multiple environments end up being identical, or, things could be structured such that multiple environments could share a Dockerfile by taking advantage of unique Docker build argument values per environment.

This PR allows for specifying a custom Dockerfile path for each environment via a new optional dockerfile field in the Vapor manifest file.

This will allow developers to incorporate their preferred filename convention for Dockerfiles, store Dockerfiles in subdirectories, and share Dockerfiles between environments.

If the dockerfile field is not specified for an environment, the current {environment}.Dockerfile filename convention is preserved.