Merge pull request moby#11203 from rhatdan/Dockerfile

We had some testers who found a hard to diagnose bug in Dockerfile

docs/man/Dockerfile.5.md

@@ -97,6 +97,9 @@ A Dockerfile is similar to a Makefile.
   exec form makes it possible to avoid shell string munging. The exec form makes
   it possible to **RUN** commands using a base image that does not contain `/bin/sh`.
 
+  Note that the exec form is parsed as a JSON array, which means that you must
+  use double-quotes (") around words not single-quotes (').
+
 **CMD**
   -- **CMD** has three forms:
 
@@ -120,6 +123,9 @@ A Dockerfile is similar to a Makefile.
   be executed when running the image.
   If you use the shell form of the **CMD**, the `<command>` executes in `/bin/sh -c`:
 
+  Note that the exec form is parsed as a JSON array, which means that you must
+  use double-quotes (") around words not single-quotes (').
+
   ```
   FROM ubuntu
   CMD echo "This is a test." | wc -