Support Dockerfiles instead of Docker Images for workspaces

[meysholdt]

It is already possible to configure a custom Docker Image for a project by using a .gitpod file.

To create a custom Docker Image, however, it is currently necessary to host the image in a public docker registry.

It would be more convenient if one could specify a Dockerfile via .gitpod file and Gitpod would on-the-fly create the Docker Image from it when needed.

This feature was requested here.

[geropl]

We just deployed this feature to https://gitpod.io. See here for an introduction, or head directly to the docs.