diff --git a/docker/fix-permissions b/docker/fix-permissions
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--- /dev/null
+++ b/docker/fix-permissions
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+#!/bin/bash
+# Set permissions on a directory
+# After any installation, if a directory needs to be (human) user-writable, run this script on it.
+# It will make everything in the directory owned by the group ${NB_GID} and writable by that group.
+# Deployments that want to set a specific user id can preserve permissions
+# by adding the `--group-add users` line to `docker run`.
+
+# Uses find to avoid touching files that already have the right permissions,
+# which would cause a massive image explosion
+
+# Right permissions are:
+# group=${NB_GID}
+# AND permissions include group rwX (directory-execute)
+# AND directories have setuid,setgid bits set
+
+set -e
+
+for d in "$@"; do
+    find "${d}" \
+        ! \( \
+            -group "${NB_GID}" \
+            -a -perm -g+rwX \
+        \) \
+        -exec chgrp "${NB_GID}" -- {} \+ \
+        -exec chmod g+rwX -- {} \+
+    # setuid, setgid *on directories only*
+    find "${d}" \
+        \( \
+            -type d \
+            -a ! -perm -6000 \
+        \) \
+        -exec chmod +6000 -- {} \+
+done
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