Add some information about the docker build return code

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

docs/sources/reference/commandline/cli.md

@@ -525,6 +525,29 @@ on your local host, and then this is sent to the Docker daemon as the
 context.  This way, your local user credentials and VPN's etc can be
 used to access private repositories.
 
+### Return code
+
+On a successful build, a return code of success `0` will be returned.
+When the build fails, a non-zero failure code will be returned.
+
+There should be informational output of the reason for failure output
+to `STDERR`:
+
+```
+$ docker build -t fail .
+Sending build context to Docker daemon 2.048 kB
+Sending build context to Docker daemon
+Step 0 : FROM busybox
+ ---> 4986bf8c1536
+Step 1 : RUN exit 13
+ ---> Running in e26670ec7a0a
+INFO[0000] The command [/bin/sh -c exit 13] returned a non-zero code: 13
+$ echo $?
+1
+```
+
+### .dockerignore file
+
 If a file named `.dockerignore` exists in the root of `PATH` then it
 is interpreted as a newline-separated list of exclusion patterns.
 Exclusion patterns match files or directories relative to `PATH` that