Experiment: Docker extension

[Lewiscowles1986]

This is just me kicking the tyres on how easy / hard it is to create a docker-desktop extension from an existing docker image.

Luckily MailHog has a UI project which is a lot of static assets, I assume will work using it's UI. We'll see.

Update:

It works

• It seems to work best with an SVG logo image, so I may need to do one of those for MailHog.
• I had to open the ports, not just expose them. Unsure why Localstack doesn't need to do this.
• The UI needed a frontend variable set. Because the port for WebUI is fixed for this, that made things simpler.
• Just used the public 8025 port; localstack told me Docker had asked them to not expose their localstack image (separate to extension image for them, I just modified my shipped image in this branch)