README: instructions for "no space left on device"

With default settings on MacOS it is relatively common to run into "no
space left of device" errors when building multiple containers. Add
instructions on what to do in those situations.

Signed-off-by: Gerwin Klein <[email protected]>

README.md

@@ -187,3 +187,44 @@ Internally, the seL4 CI system will, once a week, attempt to build the docker im
 -->
 
 [1]: https://hub.docker.com/u/trustworthysystems
+
+## Troubleshooting Docker "no space left on device"
+
+If you are building the docker images in this repository and are seeing error
+messages such as `no space left on device`, especially on MacOS, these are the
+things you can do:
+
+* On MacOS, the default overall disk space allocated to Docker is relatively
+  small and it needs to fit all images, containers, and volumes. The larger
+  CAmkES images are around 14GB in size, and if you have multiple of them the
+  numbers add up quickly. Increasing the size of the overall disk allocation to
+  somewhere in the vicinity of 128GB is usually enough to fix the problem. To do
+  so, in the Docker Desktop app, go to Settings (gear box at the top right in
+  version 4.29, for instance), go to Resources, scroll down to find "Virtual
+  disk limit", and increase to 128GB or more.
+
+* On Linux, Docker does not have a bound on disk space, but you might find
+  overall disk space on the host to be low. You can either free up disk space
+  elsewhere on the host, or try to free up Docker resources.
+
+* Freeing up unused Docker resources (Linux and MacOS). The following steps free
+  up space in increasing order of aggressiveness. If you just want to free all
+  of it, skip to the last step.
+
+      # free up all dangling images and all stopped containers
+      docker system prune
+
+  This is relatively safe in that it won't remove images you might still want to
+  use. You can list current docker images using the command `docker image ls`
+  and remove specific ones with `docker image rm <image>`. To remove all images
+  not attached to a currently running container, you can
+
+      # remove all images
+      docker image prune -a
+
+  The wipe-all option to remove everything that is not currently in use by a
+  running container is:
+
+      # remove everything not currently in use
+      docker system prune -a
+      docker volume prune -a