set some good memory defaults

[bacongobbler]

Some good starting points:

work_mem: Typically 32M (< 5G RAM) or 64M (>= 5G)
maintenance_work_mem: 128M (< 5G RAM) or 256M (>= 5G)
shared_buffers: min(0.25 * RAM, 17.5G)
effective_cache_size: 0.8 * RAM

[bacongobbler]

we won't be able to determine what available resources are there for postgres to consume, so we'll have to document how to tune these settings when extending this image.