Dockerize GPM

[dhiguero]

The current implementation only supports a dockercmd provider that executes several docker run commands to generate the protos using an open source third party docker image. It will be useful for some scenarios such as CI pipelines to provide a single docker image that embeds both the GPM command and the required gRPC generation helpers.

Pending tasks:

• Analyze the third party image to determine integration points.
• Create a base docker file embedding GPM from the previous one.
• Define a new entrypoint for the image and document require environment variables and mounting paths.
• Implement new proto generator implementation