WIP: Squash docker layers after building

[zootalures]

This is a quick POC to demonstrate doing docker-daemon level image squashing This will change the behaviour of image pulls so should be reviewed w.r.t. actual system performance before merging.

[shaunsmith]

Two issues @owencliffe. The native images don't pass on the build script args to the docker build command so passing "--squash" won't have any effect. The build-native script expects the first arg to be the runtime dir. You've made the first arg "--squash" which will result in failure.

I've got a PR to update the native image builds pending: #246