Dockerfile for testing Ubuntu 20.04

[jkbecker]

Here is a minimal docker build using Ubuntu 20.04.

It is derived from the ubuntu_1804_py3k image, i.e., it uses Python 3 by default.

Builds fine for me, but only after adding an apt-get install -qq -y pkg-config line. Otherwise, installation of gr-osmosdr fails during the installation of airspy. Not sure if this should be upstreamed into the airspy.lwr as a dependency? Some context:

• just installing pybombs install airspy on a fresh Ubuntu 18.04 will fail with the same issue, however installing pybombs install gr-osmosdr works fine (probably because pkg-config gets installed somewhere during the tree installation before airspy gets installed
• Not sure how deterministic tree resolution is, but the previous point sounds to me like pkg-config should be listed in the airspy recipe dependencies?

I have added fixes to that recipe (see gnuradio/gr-recipes#195 / gnuradio/gr-recipes#196), but I'm unsure if airspy was a lucky find or if other recipes may also fail if this is removed (didn't try them all individually, let me know if you'd like me to).

[argilo]

Adding pkg-config as a dependency in the recipes that need it seems like the right approach, so let's do that. The airspy recipe now has pkg-config and a dependency and I'm testing this pull request without the RUN apt-get install -qq -y pkg-config line to see how it goes.

[argilo]

After that change, things work without pkg-config in the dockerfile. I'll merge this and then remove it in a follow-up commit.

[argilo]

pkg-config was removed in b3c02fd975e3de392dae2c7f06faa00044b68a52.