Dockerfile: Make more resilient

[mchack-work]

• Can we use Alpine or similar small distro as base instead of Ubuntu?

• We might be able to use more tools directly from distro packages instead of from upstreams. Investigate, for instance, iverilog and verilator.

• Not very happy with the code patching of nextpnr, even if it's small, in the Dockerfile. Alternatives? Keep a fork of our own of nextpnr? Anything else?

• Replace at least some of the git clone commands with actual release tar balls. Include digests to see that the downloaded file is what we expected.

[dehanj]

Not very happy with the code patching of nextpnr, even if it's small, in the Dockerfile. Alternatives? Keep a fork of our own of nextpnr? Anything else?

I guess the options we have is, in some form of sorted list where 1 is the "best" option:

1. upstream the changes
2. figure out if it is really needed, is there any workarounds we can use outside of the specific software?
3. If the change is small enough: patch in dockerfile
4. fork the project and maintain