Fix Docker build directory visibility (Netflix#37)

* Fix Docker build directory visibility

The project Dockerfile lives in a `./docker` directory, but references
files hosted in the root directory, namely:

- `.nvmrc`
- `setup.py`

Dockerfiles can only reference files in the same folder as the build
context. This results in build errors reported in
Netflix/dispatch-docker#2 and initially
addressed in Netflix/dispatch-docker#1

```
ERROR: Service 'web' failed to build: COPY failed: stat
/var/lib/docker/tmp/docker-builder448268992/.nvmrc: no such file or
directory
```

This is a simple fix to move the Dockerfile into a reasonable scope via
symlink. However, open source contributors cannot build the image
locally since the build steps appear rely on access to resources on
Netflix internal network.

* Update installation.md

Co-authored-by: kevgliss <[email protected]>

Author: TheDahv

Dockerfile

@@ -0,0 +1 @@
+docker/Dockerfile

docs/installation.md

@@ -10,6 +10,19 @@ Dispatch relies on multiple services to work, which are all orchestrated by `Doc
 - At least 2400MB memory
 - 2 CPU Cores
 
+## Building a Docker Image
+
+The Netflix team builds Docker images to run Dispatch. To run locally, run the
+following from the project root:
+
+```
+docker build -t docker-local .
+```
+
+This is only useful for local development purposes, as you should use the
+published Docker image described in the next section.
+
+
 ## Installing Dispatch Server
 
 We strongly recommend using Docker, for installing Dispatch and all it's services. If you need to to something custom, you can use this repository as the basis of your setup. If you do not wish to use the Docker images we provide, you can still find Dispatch on PyPI. However, we don't recommend that method. You'll need to work your way back from the main Dispatch image. It is not too hard, but you are likely to spend a lot more time and hit some bumps.