feat(docs): Add new docker-compose and tag instructions

Author: kylef

docs/integrations/docker.md

@@ -4,6 +4,15 @@ We provide a [swiftenv image for
 Docker](https://hub.docker.com/r/kylef/swiftenv/). You can pull it down to use
 Swiftenv and Swift in Docker or base your own images from the swiftenv image.
 
+Swiftenv provides multiple base docker images:
+
+- `latest` - Image with swiftenv and all runtime dependencies to use Swift binaries.
+- `build` - Image with swiftenv and all build dependencies to be able to build Swift from source.
+- `swift3` - Image with swiftenv and the latest stable version of Swift 3.
+- `swift` - Image with swiftenv and the latest stable version of Swift.
+
+All of the docker images are based on top of Ubuntu 16.04 LTS (Xenial).
+
 ## Running the swiftenv image directly
 
 You can pull down the `kylef/swiftenv` docker image and run it.
@@ -15,18 +24,55 @@ $ docker run -i -t --entrypoint /bin/sh kylef/swiftenv
 swiftenv 1.2.0
 ```
 
-Using the image, you can install any version of Swift you like and use it.
+Or for swiftenv with latest Swift:
 
 ```shell
-# swiftenv install 3.0
+$ docker pull kylef/swiftenv:swift
+$ docker run -i -t --entrypoint /bin/sh kylef/swiftenv
+# swift --version
+swift 3.0.1
 ```
 
 ## Building a docker image using swiftenv
 
 You may base your own Docker image from the swiftenv image, you may then
-install any Swift version you desire.
+install any Swift version you desire in your container.
 
 ```
 FROM kylef/swiftenv
 RUN swiftenv install 3.0
 ```
+
+## `docker-compose`
+
+Docker compose allows you to setup and run your project easier. It's a wrapper
+around `docker`.
+
+For example, we can create a service called `commander` on top of the swift3
+swiftenv image which maps the source files into the docker container.
+
+```yaml
+version: '2.0'
+
+services:
+  commander:
+    image: kylef/swiftenv:swift3
+    volumes:
+      - './Sources:/code/Sources'
+      - './Tests:/code/Tests'
+      - './Packages:/code/Packages'
+      - './Package.swift:/code/Package.swift'
+    working_dir: /code
+    command: swift build
+```
+
+We can then use `docker-compose` to run commands such as `swift test` inside
+our container.
+
+```shell
+$ docker-compose run commander swift test
+```
+
+You can switch out the `image` line of your service for `build: .` to build a
+`Dockerfile` found in your repository instead of going straight from the
+`swift3` image. This allows you to pin to a specific version of Swift.