Add Dockerfile for Gitpod

[jelaniwoods]

By using a Dockerfile in each project, we can pre-bundle the gems for each project. We also aren't installing multiple versions of gems in the image that was causing some issues with firstdraft/appdev#216.

The requested changes are:

• Add a Dockerfile to the root folder of each project.
• In the gitpod.yml use the Dockerfile instead of a specific image.

The one hangup:

• I can't access the yarn command when building the image. This is strange because yarn should definitely be installed in the base image, since it's not like we had installed it before.

You can open this branch of photogram-signin in Gitpod to see the changes. Open in Gitpod

[pmckernin]

Thank you for explaining this to me at Harper, makes sense and LGTM

[jelaniwoods]

Closing in favor of #130