docker: switch current working directory to a writeable directory

In Sync Gateway 3.1.2 on github standard configuration, it is not possible to start a database without switching the working directory of the image, due to CBG-3584 which writes files the the current working directory.

This is due to an unspecified working directory of the image / but it is run as the sync gateway user.

I recommend changing the working directory to the same directory we use for systemd files, which I think is /home/sync_gateway. Setting this directory when running the container will allow SG to run.

This is a regression in 3.1.2 (from 3.1.1) in ability to load a database.