Add Docker example

Recursiveness.md

@@ -33,5 +33,12 @@ To check files whose shebang indicate that they are sh/bash/ksh scripts:
 
 ```
 # POSIX
-find /path/to/scripts -type f -exec grep -Eq '^#!(.*/|.*env +)(sh|bash|ksh)' {} \; | xargs shellcheck
+find /path/to/scripts -type f -exec grep -Eq '^#!(.*/|.*env +)(sh|bash|ksh)' {} \; \
+  | xargs shellcheck
+```
+
+## With Docker
+
+```
+docker run --volume /path/to/scripts:/mnt koalaman/shellcheck-alpine sh -c "find /mnt -name '*.sh' | xargs shellcheck"
 ```