Fix docker build

[gkreitz]

I had apparently been a bit sloppy in properly testing docker building, update_docker.sh failed on git clone as I'd changed ' to " between testing and committing.

• Fixes silly error in docker files, where we due to a missing && attempted to delete the files apt-get and update instead of running the command. :)
• Adds a -d option to update_docker.sh which allows the repository to be dirty, simplifying testing one more time before commit:ing.