add docker image documentation

docs/distributions/docker.md

+# Docker images
+
+## Introduction
+
+We currently build and maintain docker images for supported distributions.
+This is currently SLC6 and CC7
+
+|distribution|gitlab repo|docker hub url|maintained|
+|--|--|--|--|
+|slc5-base |https://gitlab.cern.ch/linuxsupport/slc5-base|https://hub.docker.com/r/cern/slc5-base|No longer maintained - last release 2019.07.24 |
+|slc6-base |https://gitlab.cern.ch/linuxsupport/slc6-base|https://hub.docker.com/r/cern/slc6-base|Maintained|
+|cc7-base |https://gitlab.cern.ch/linuxsupport/cc7-base|https://hub.docker.com/r/cern/cc7-base|Maintained|
+
+## Building a new image
+
+1. Build an image via koji, using the image-build command from the README.md file of the gitlab $distribution repository, this will build a .tar.xz file
+2. Download the .tar.xz file from koji and import into your local docker installation `docker load < $filename.tar.xz`
+3. Test the image `docker run -it $imagename /bin/bash`
+4. Providing testing is okay, update the 'latest image' list in README.md of the gitlab $distribution repo
+
+## Uploading the new image to the CERN gitlab registry
+
+1. Push the image to gitlab
+    1. `docker login gitlab-registry.cern.ch`
+    2. `docker tag $imageid gitlab-registry.cern.ch/linuxsupport/$distribution:$date_YYYYMMDD`
+    3. `docker tag $imageid gitlab-registry.cern.ch/linuxsupport/$distribution:latest`
+    4. `docker push gitlab-registry.cern.ch/linuxsupport/$distribution`
+
+## Uploading the new image to the hub.docker.com registry
+
+1. Push the image to dockerhub
+    1. `docker login --username=$username`
+    2. `docker tag $imageid cern/$distribution:$date_YYYYMMDD`
+    3. `docker push cern/$distribution`
+2. Update the description of $distribution on dockerhub, if necessary
+
+Note - we do not by default tag each new image as 'latest' on hub.docker.com, instead we wait for a period of time whilst the image is tagged as 'latest' in the internal gitlab registry to ensure that there are no regressions. This period of time is typically 4 weeks. After this time has past, we can safely tag the last image uploaded on hub.docker.com as 'latest'.