Dockerize benchmarks

[peterstace]

The benchmarks have dependencies on a particular version of GEOS.

Rather than requiring developers to install that version of GEOS, the benchmarks should be dockerized instead.

[peterstace]

I've attempted to dockerise the whole script, but that doesn't work too well -- since we need to switch between branches in the script.

I'll instead attempt to build two images containing just the appropriate benchmark binaries, and then run them with the script.