Update book

book/src/docker.md

@@ -0,0 +1,19 @@
+# Docker Guide
+
+This repository has a `Dockerfile` in the root which builds an image with the
+`lighthouse` binary installed.
+
+To use the image, first build it (this will likely take several minutes):
+
+```bash
+$ docker build . -t lighthouse
+```
+
+Once it's built, run it with:
+
+```bash
+$ docker run lighthouse lighthouse --help
+```
+
+_Note: the first `lighthouse` is the name of the tag we created earlier. The
+second `lighthouse` refers to the binary installed in the image._