Describe docker requirement for test

doc/build.md

@@ -3,7 +3,7 @@ How to build and run from the source tree
 
 First of all, Install [sbt](http://www.scala-sbt.org/index.html).
 
-```
+```shell
 $ brew install sbt
 ```
 
@@ -12,7 +12,7 @@ Run for Development
 
 If you want to test GitBucket, type the following command in the root directory of the source tree.
 
-```
+```shell
 $ sbt ~jetty:start
 ```
 
@@ -25,7 +25,7 @@ Build war file
 
 To build war file, run the following command:
 
-```
+```shell
 $ sbt package
 ```
 
@@ -33,7 +33,7 @@ $ sbt package
 
 To build an executable war file, run
 
-```
+```shell
 $ sbt executable
 ```
 
@@ -41,8 +41,15 @@ at the top of the source tree. It generates executable `gitbucket.war` into `tar
 
 Run tests spec
 ---------
+Before running tests, you need to install docker.
+
+```shell
+$ brew cask install docker       # Install Docker
+$ open /Applications/Docker.app  # Start Docker
+```
+
 To run the full series of tests, run the following command:
 
-```
+```shell
 $ sbt test
 ```