Make it clear who is doing variable expansion

Add sentece to RUN, CMD, and ENTRYPOINT exec sections making it clear
that it is the shell doing the environment variable expansion.

Signed-off-by: David Dooling <dooling@gmail.com>

docs/reference/builder.md

@@ -541,6 +541,9 @@ RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
 > `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
 > If you want shell processing then either use the *shell* form or execute
 > a shell directly, for example: `RUN [ "sh", "-c", "echo $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
 >
 > **Note**:
 > In the *JSON* form, it is necessary to escape backslashes. This is
@@ -607,6 +610,9 @@ instruction as well.
 > `CMD [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
 > If you want shell processing then either use the *shell* form or execute
 > a shell directly, for example: `CMD [ "sh", "-c", "echo $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
 
 When used in the shell or exec formats, the `CMD` instruction sets the command
 to be executed when running the image.
@@ -1075,6 +1081,9 @@ sys	0m 0.03s
 > `ENTRYPOINT [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
 > If you want shell processing then either use the *shell* form or execute
 > a shell directly, for example: `ENTRYPOINT [ "sh", "-c", "echo $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
 
 ### Shell form ENTRYPOINT example