The Dockerfiles had bitrotted some.
Get them passing again, add libpcre2-dev where we can so we aren't
hitting more servers than necessary, and reformat the bionic files so
they can share more of the same image.
This is the start of an effort to make it easier to build and run tests in
various Linux environments. The idea is to reduce our reliance on CI and
also allow an easy to way capture tricky environments like musl or gcc 5.
This adds two initial Dockerfiles corresponding to Ubuntu Bionic, and
Ubuntu Bionic with Thread Sanitizer enabled. It also adds a new script
`docker/docker_run_tests.sh`. An example of usage:
docker/docker_run_tests.sh docker/bionic-tsan.Dockerfile
When run, this builds a Docker image (which is cached after the first
build) and sets its entry point to a new script `fish_run_tests.sh`. It
then launches a container with that image, with a directory `/fish-source`
bound to the fish-shell source directory on the host. Note it is a bind
mount, not a copy, so changes to host files are instantly visible inside
the container. It then configures with CMake and runs the tests.
The Docker user is `fishuser` with password `fish`.
The script also supports two arguments `--shell-before` and
`--shell-after`. These drop the user into a bash shell before (or after)
the tests are run, to aid in debugging.
Note there's no automation for invoking this script yet; it must be run
manually. But it runs on both Mac and Linux!
