add a documentation note on backslash usage in shell form of RUN
@@ -311,6 +311,12 @@ commands using a base image that does not contain `/bin/sh`.
> If you want shell processing then either use the *shell* form or execute
> a shell directly, for example: `RUN [ "sh", "-c", "echo", "$HOME" ]`.
+> **Note**:
+> If you choose to use the *shell* form, any time you want to continue a single
+> `RUN` instruction onto the next line, it has to be ended with a backslash `\`.
+> For example, `RUN /bin/bash -c 'source $HOME/.bashrc ;\` then on the next
+> line ` echo $HOME '`.
+
The cache for `RUN` instructions isn't invalidated automatically during
the next build. The cache for an instruction like
`RUN apt-get dist-upgrade -y` will be reused during the next build. The
