Fix documentation on docker image to say you must build before running (#1987)

* Can not run before building

unless you provide a library/powershell image

* Update docker.md

docs/installation/docker.md

@@ -1,15 +1,6 @@
 Docker
 ======
 
-If you are using Docker, there is a very simple way to try PowerShell:
-
-```
-docker run -it powershell
-```
-
-This Docker image is based on Ubuntu 16.04, and follows the instructions from the [Linux Installation docs][u16].
-[u16]: linux.md#ubuntu-1604
-
 Build
 =====
 
@@ -18,3 +9,14 @@ If you want to build it yourself:
 docker build -t powershell .
 ```
 
+Run
+===
+
+After you have built using Docker, running PowerShell very simple:
+
+```
+docker run -it powershell
+```
+
+This Docker image is based on Ubuntu 16.04, and follows the instructions from the [Linux Installation docs][u16].
+[u16]: linux.md#ubuntu-1604