Add usage of strict shell (rstacruz#1841)

Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

dockerfile.md

@@ -47,6 +47,20 @@ ADD file.xyz /file.xyz
 COPY --chown=user:group host_file.xyz /path/container_file.xyz
 ```
 
+### Run commands in strict shell
+
+```docker
+ENV my_var
+SHELL ["/bin/bash", "-euo", "pipefail", "-c"]
+
+# With strict mode:
+RUN false # fails build like using &&
+RUN echo "$myvar" # will throw error due to typo
+RUN true | false # will bail out of pipe
+```
+
+Using `shell` will turn on strict mode for shell commands.
+
 ### Onbuild
 
 ```docker