Switch to Ubuntu "slim"

[utdrmac]

Ubuntu announced "slim" versions of Ubuntu based on 18.04. Per their announcement,
https://ubuntu.com/blog/minimal-ubuntu-released, if you specifically pull ubuntu:18.04, then you automatically get the slim image.

Several minor modifications to merge some build stages.

$ docker build -t checker -f Dockerfile-ubuntu .
...
$ docker images
REPOSITORY               TAG                 IMAGE ID            CREATED             SIZE
checker                  latest              04ffd2e4b87c        3 minutes ago       445MB

The tezos binaries themselves (tezos-*) sum to 399MB. Geez that's a lot of bloat.

[utdrmac]

Couple comments: 1) seems that ubuntu:latest is the same as ubuntu:18.04. I did not realize that.

$ docker images | grep ubuntu
ubuntu                   18.04               775349758637        5 weeks ago         64.2MB
ubuntu                   latest              775349758637        5 weeks ago         64.2MB

2. If you add RUN strip tezos-* after the make build-deps/eval/make line, this will remove debugging symbols from all binaries and save about 100MB.

utdrmac/tezos             carthage_b1a7b92b   7a0d6a9c84ae        26 minutes ago      342MB

[utdrmac]

You can't use bubblewrap within docker containers to create new namespaces (unless you run docker with more privileges) so we set up opam without sandboxing. Because we don't use sandboxing, installing bubblewrap is pointless.