Move a few things around

- move runtime dep install into a `RUN` line since they stick around (so we can have that get cached and speed up rebuilds)
- add `/usr/local/nginx/sbin` directly to our path instead of symlinking the `nginx` binary into `/usr/local/sbin` in case future versions have multiple useful binaries
- note why we use Jessie instead of Wheezy so we don't go down that road again :)