Fix "Read-only file system" error in dev (again)

Looks like the `-perm /u=w` argument incorrectly includes files from the
read-only filesystem. This should be fixed by the `-writable` option.

Close #392

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix "Read-only file system" errors when running `tutor dev runserver` commands on Mac OS. (again, see #392)
 - [Feature] Automatically bind-mount volumes from the `volumes/` directory with the `--volume=/...` syntax.
 
 ## v11.0.7 (2021-01-11)

tutor/templates/build/openedx-dev/bin/docker-entrypoint.sh

@@ -7,7 +7,7 @@ if id -u openedx > /dev/null 2>&1; then
     find /openedx \
         -not -path "/openedx/edx-platform/*" \
         -not -user openedx \
-        -perm /u=w \
+        -writable \
         -exec chown openedx:openedx {} \+
     echo "File permissions set."