Specify ENV variables are also used for CMD.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

docs/man/Dockerfile.5.md

@@ -120,7 +120,8 @@ or
 **ENV**
  --**ENV <key> <value>**
  The ENV instruction sets the environment variable <key> to
- the value <value>. This value is passed to all future RUN instructions. This is
+ the value <value>. This value is passed to all future 
+ RUN, ENTRYPOINT, and CMD instructions. This is
  functionally equivalent to prefixing the command with **<key>=<value>**.  The
  environment variables that are set with ENV persist when a container is run
  from the resulting image. Use docker inspect to inspect these values, and

docs/sources/reference/builder.md

@@ -349,7 +349,8 @@ accessible from the host by default. To expose ports to the host, at runtime,
     ENV <key>=<value> ...
 
 The `ENV` instruction sets the environment variable `<key>` to the value
-`<value>`. This value will be passed to all future `RUN` instructions. This is
+`<value>`. This value will be passed to all future 
+`RUN`, `ENTRYPOINT`, and `CMD` instructions. This is
 functionally equivalent to prefixing the command with `<key>=<value>`
 
 The `ENV` instruction has two forms. The first form, `ENV <key> <value>`,