Dedicated support for Startup Command.

[isaacabraham]

This PR closes #999

The changes in this PR are as follows:

• Adds a startup_command keyword to the Web App builder.

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

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

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

webApp {
    startup_command "foo.exe"
}

I'll test it end-to-end shortly. I'm sure I've done Linux deployments in the past so I'm surprised I can't find a clear way of doing this - @ninjarobot do you know any way to already do this in Farmer?

[isaacabraham]

Yes, similar to that. The WebApp builder already lets you supply a Docker container and Azure Container registry path, plus the startup command for docker - what I think wasn't available previously was the ability to set the startup command if you just uploaded your app using Zip Deploy (HTTP upload) - in that case, App Service creates a docker container for you with your app, but doesn't know what to use for the startup command so it just sits there doing nothing.

[isaacabraham]

@ninjarobot Ok I've remembered how you do it on Linux Web Apps again. If you supply a web.config file that contains the startup path, the app service uses that to do it. So this PR isn't really necessary.

[ninjarobot]

@isaacabraham do you still want to merge this?

[ninjarobot]

@isaacabraham I'm not sure whether you want to include this or not since you figured out another way. Do you want to merge this feature?

[isaacabraham]

I don't think it's really needed any more although it doesn't hurt to merge in.