Docker & Docker Compose support

Hi there!

## Rationale

I think that nowadays, containers have been embraced by many developers and that it would be nice to have some support for Docker inside the starter template by providing two files.

## Implementation

More details [here](https://github.com/gridsome/gridsome/issues/58#issuecomment-569505026).

## Pros

1. Docker users will now embrace the full power of they favorite tool without having to think about it because it will be provided directly inside the starter.
1. Non-Docker users won't have to change anything since this solution does not impact any other files in the starter.
1. Docker officially supported by Gridsome without much effort and means more users willing to take the leap and try it out (this can include system administrators that want to create nice little web interfaces without the hassle of having to know and install all the development tools that we are used to).

What do you think? In my opinion this is a frictionless solution that I could easily provide in the form of a Pull Request if accepted.

Read the implementation details in the linked Gridsome issue comment, then inspect the starter repository root to determine where the two requested files belong. Done means Docker and Docker Compose support is provided without requiring changes to the other starter files.

Written by the indexing model from the issue text.