Don't build/push new desktop app if nothing's changed

[Webreaper]

Perhaps separate Damselfly.Desktop into a separate project/build that gets pushed into the image only if it's changed. That would make docker pulls much more efficient, as 90% of what gets downloaded is the rebuilt desktop app each time.

Alternatively, hash/checksum the app, and only include it if something's changed. Would need to separate versions though, as the app gets rebuilt just on a version bump and its signature would change.