Fix latest wal-e build missing XML library headers
Fix README example cron job
Change cron and Postgres execution to use wrapper scripts

Reason being that host-mounted volumes may not mount with the correct
permissions, so before running we need to chown and chmod the
directories.
Simplify example WAL-E `docker run` in README
Fix example crontabs in README
Switch base image to phusion/baseimage

Useful to easily get a sane install of cron, syslog, and runit
Add "Basic usage" section to README