Add curl to the installed dependecies for the Dockerfile to allow easier scripting

[ironcrafter54]

By adding curl to the packages installed in the docker image published it would make it simpler to create simple shell scripts that sent web hooks to other programs such as home assistant to update them on the state of the shairport-sync instance. To whom it concerns I have tested this and it works without issue, happy to create a pr to add this if it aligns with the project goals.

[mikebrady]

Thanks for the suggestion, which has been implemented in the development branch. If you got a chance to check it and report back, it would be appreciated.