Mention docker mount flags in the Readme

Author: vvolkl

README.md

@@ -43,3 +43,7 @@ To avoid any overhead on the CernVM-FS stratum 1 servers that you are accessing,
 ## Limitations
 
 This GitHub Action is only expected to work in workflows that [run on](https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on) ubuntu targets (and even then likely only `ubuntu-latest`). This exludes the `macos` and `windows` targets.
+
+## Use With Docker
+
+In case your workflow uses docker containers, the cvmfs directory can be mounted inside the container by using the flag `-v /cvmfs:/cvmfs:shared`.