Update ampinstmgr.service for docker

[BroOtti]

If docker.service is present, ampinstmgr.service should only start after docker.service is running. This way ampinstmgr.service is also stopped before docker.service is stopped on shutdown which allows a graceful stop.

This also helps to solve a part of issue #1019

[Greelan]

This seems a reasonable change. But I am not sure that the files in this GitHub are ultimately used as a source for the units included with ampinstmgr itself.

[BroOtti]

At least it's visible here for others and changes can be made where they need to be made for this :D Maybe @IceOfWraith or @PhonicUK can tell us more or look into this ^^

[Greelan]

Right, just saying it probably wouldn't have much visibility here otherwise