Add prebuild, postbuild & postinstall hooks

[aedm]

If there is a docker-preinstall.sh next to the Dockerfile, MiniMeteor should run it as a build step before meteor build. Similarly, docker-postbuild.sh should optionally run after meteor build, and docker-postinstall should run after npm install.

This way users can install additional apt packages and execute custom build steps.

[awatson1978]

Use case: Running meteor platform-remove ios so Mac binaries are bundled into a meteor app before being put into a docker image that will run on AWS Linux.