Dependencies for a pure artifact server are undocumented

Background

We document an alternate install path for artifact-server-only BuildStream here, but we do not list the dependencies of the server.

Task description

We should document the dependencies of the artifact server, since they will differ from main BuildStream. In particular, I believe neither fuse nor bubblewrap are required.

• Figure out the actual dependencies of the artifact server
• Ensure that the installation path works without unnecessary dependencies
• Document the new list of dependencies
• Update the relevant docker image

Acceptance Criteria

We should have a clearly documented set of dependencies for the artifact server.

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