Build fix lalsuite from src

Updates the cmake build instructions such that they use pkg-config to search for e.g. LAL libraries. Provided PKG_CONFIG_PATH is correctly set (usually achieved via source lalsuiterc), users should now be able to build BayesWave against local installations of LALSuite built from source and installed to e.g. /home/user/opt/lscsoft/lalsuite etc, which is potentially useful for development against bleeding-edge waveforms etc.

Does not affect users' installations using system-installed packages.

I've also added an additional Dockerfile which can be used (manually for now) to test this process.