Local development scripts

Two scripts are provided to help run mulitple projects together on your local dev machine. The first `script/build-dev`, builds the immutant-docker container with the name "immutant-dev" (a name used in our synapse configs). You'll only need to run this initially, and then any time the immutant-docker project is changed.

The other `script/build-subproject` builds an immutant project and then runs the immutant-docker container with all immutant projects it can find. It does this by using the `--cidfile` flag with `docker run` on the subprojects it builds. The script simply looks for files named `immutant-volume.cid` in sibling directories and mounts the containers discovered.

(Since the `immutant-volume.cid` files live in their respective project's directories, those files should be ignored by version control.)

When you run `script/build-subproject` you'll need to provide the the subproject's name and the environment to run in. For example:

```sh
./script/build-subproject nixon development
```