Enable the docker image tag to be specified for a slot.

[ninjarobot]

This PR closes #1005

The changes in this PR are as follows:

• WebApps: Enable the docker image tag to be specified for a slot.

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

• [x] 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.

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:

webApp {
    name "my-webapp-2651A324"
    sku (Sku.Standard "S1")
    // Production using one image tag.
    docker_image "nginx:1.22.1" ""

    add_slots
        [
            appSlot {
                name "staging"
                // The slot is staging a newer image tag.
                docker_image "nginx:1.23.1" ""
            }
        ]
}

[ninjarobot]

@forki this will allow you to set the container image for a slot so that you can switch to a container from a new tagged image by swapping slots. Please give it a review and let me know if this meets your needs for #1005.

The production site:

The staging slot:

[ninjarobot]

@isaacabraham here's a fix for the issue @forki brought up recently.