Merge pull request #16253 from AmeBel/update-documentation-on-run

add a documentation note on backslash usage in shell form of RUN

docs/reference/builder.md

@@ -295,6 +295,17 @@ any point in an image's history, much like source control.
 The *exec* form makes it possible to avoid shell string munging, and to `RUN`
 commands using a base image that does not contain `/bin/sh`.
 
+In the *shell* form you can use a `\` (backslash) to continue a single
+RUN instruction onto the next line. For example, consider these two lines:
+```
+RUN /bin/bash -c 'source $HOME/.bashrc ;\
+echo $HOME'
+```
+Together they are equivalent to this single line:
+```
+RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
+```
+
 > **Note**:
 > To use a different shell, other than '/bin/sh', use the *exec* form
 > passing in the desired shell. For example,