Adding instructions to clean docker images

continuousintegration/buildtree_runner/README.md

@@ -55,6 +55,11 @@ cd ~/CTA/continuousintegration/buildtree_runner
 
 This image also contains embedded scripts from the CTA tree. As the build is 2 stage, the embedding of scripts can be run separately, saving the time to install the RPMs.
 
+When updating image content, be careful that some layers can be cached event if the content changed (for example an `ADD` layer may not be updated in the new image...). To make sure everything is clean, you need to remove all cached layers from `docker`:
+```
+docker rmi $(docker images -a | tail -n+1 | awk '{print $3}')
+```
+
 ### Preparing the environment (MHVTL, kubernetes volumes...)
 
 MHVTL should then be setup by running: