Improve docker image tagging

[JasonYeMSFT]

We have two kinds of image tags right now:

• latest The most recently published image has this tag.
• Exact version (e.g. 1.0.1, 2.0.0-beta.9). The image at the exact version.

Our remote hosting templates are written to pull image with the latest tag because we don't want to hardcode a specific version in a long lasting template. This works most of the time but will have problem when we publish 1.0.x patch releases. 1.0.x doesn't have the code for remote hosting. When a 1.0.x patch release is published, the image with the latest tag will be pulled by the templates and fail to run as a remote server.

We can introduce additional tags to better categorize the images. For example, a v1-latest (or just v1) tag for the latest GA version and a v2-latest tag. This would solve this problem and allow automation systems to pull the latest images with desirable major version.

[joshfree]

@hallipr could you assist with this 2nd docker issue?