Add docker port setting for app service

[stevebelskie]

This PR closes #869

The changes in this PR are as follows:

• Adds support for specifying a custom docker port for an app service

I have read the contributing guidelines and have completed the following:

• Tested my code end-to-end against a live Azure subscription.
• Updated the documentation in the docs folder for the affected changes.
• Written unit tests against the modified code that I have made.
• Updated the release notes with a new entry for this PR.
• Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

let wa = webApp {
    name "myApp"
    docker_port 8080
}

[isaacabraham]

This looks great! I'll do a proper code review this week - probably just some syntactic style changes but this looks very good.

Thank you very much.

[stevebelskie]

Happy to help! Still getting used to F# so there were a few places I wasn't sure about what would be idiomatic, but overall the code base is incredibly well laid out and it was pretty easy to figure out what goes where.

I'll definitely be contributing to (and using!) Farmer more in the future.

[ninjarobot]

Thank you, this is released now in 1.6.27.