Generate an appropriate .dockerignore file next to the Dockerfile to ignore app.yaml and other files.

[ivannaranjo]

We should generate a .dockerignore file to ignore all unnecessary files from the final Docker image. This .dockerignore file should include:

• Dockerfile
• app.yaml
• .git
• .hg
• ... and other version control system directories.

[chrsmith]

In case you were unaware, this is a good resource:
https://github.com/github/gitignore