Add Support for Airspy HF+ Discovery

[Zindswini]

These changes to the dockerfile install the dependencies required to make use of the Airspy HF devices.

This makes the spyserver image work plug and play with the Airspy HF+ Discovery (Tested). Unfortunately, this does increase the image size by quite a bit, from 103 to 371 MB.

I'm far from a docker expert, but this worked for me and I figure that this might help others with the same device. If you have a way to simplify this, please do!