Add playable tracks to gitpod

As noted in #1818 (closed), gitpod workspaces currently do not contain playable tracks. Turns out we have a repo that has some free music in it that we can import in gitpod.

I added an option to pass through VUE_APP_INSTANCE_URL variable from gitpod user settings to the docker container in case anyone wants to use a custom instance by default when creating a new workspace.

I've also added a small icon to the welcome message task in gitpod to make it more noticible.

Before:

After:

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)