docker.cmd is hard to add/modify to

[bfirsh]

It is difficult to modify docker.cmd by the consumer of the label, which meant I didn't use it for Whalebrew. See here for more details: whalebrew/whalebrew#6

Instead, I opted for specific labels for each thing I wanted to configure. Eg. the location to mount each volume, the port to expose each port as, and so on. See here: https://github.com/bfirsh/whalebrew#configuration-1

This means I can construct and add Docker options at runtime, and is much safer because it doesn't allow arbitrary options to be passed. Also, it could potentially work with rkt too.

[lizrice]

I like the way you've done that @bfirsh. Pinging @garethr @mhausenblas @amouat - wdyt, should we look at moving to this style in label-schema?