Docker build instructions

[MrCreosote]

At least for me, the docker build command in README.md didn't work:

sudo docker build -t shock:latest docker/Dockerfile 
unable to prepare context: context must be a directory: /home/[bla]/localgit/Shock/docker/Dockerfile

This does though:

sudo docker build -t shock:latest docker

docker -v
Docker version 1.9.1, build a34a1d5

[wgerlach]

I updated the README.md, thx.

[MrCreosote]

Thanks for the fix. When I tried to build 0.9.13, it looks like docker just used a cache (I'm a docker noob) and I wound up with the 0.9.12 binary. Adding --no-cache to the build command fixed that - noting that in the build instructions (assuming that's the correct fix) would be helpful for Shock users new to docker.