[FIX] Tweaking Docker setup to enable running shell commands when deployed

[gcaprio]

WHY are these changes introduced?

Fixes #100

The default Docker implementation for this template hardcodes the rails server startup within the entrypoint.sh. This disables the ability to run other commands when the container is running. For example, when deployed to Heroku, you cannot open a shell to your container.

WHAT is this pull request doing?

It moves the rails server startup command outside of entrypoint.sh and into the Dockerfile, supporting the existing behavior. However, it also allows you to override the CMD being run so you can execute shell commands as well.

It also adds a sanity check before deleting the existing server.pid in case it doesn't actually exist.

Checklist

Note: once this PR is merged, it becomes a new release for this template.

• [-] I have added/updated tests for this change
• [-] I have made changes to the README.md file and other related documentation, if applicable

[gcaprio]

thanks @Spiritaine !

[mllemango]

Thanks!