build(docker): improve Docker start-up time locally

Adds a .dockerignore file to avoid including (almost 1GB worth) of superfluous files in the Docker context. These files
are mostly the result of running the app outside of Docker (primarily the NPM dependencies and Parcel cache). On macOS
at least, this sees a dramatic speed increase.

Refs #390

.dockerignore

@@ -0,0 +1,7 @@
+/.github/
+/.parcel-cache/
+/coverage/
+/deploy/
+/dist/
+/node_modules/
+/*.sqlite3