app_containers: Move to using CONTAINER_BIND_MOUNTS for extra binds

...instead of the overly generic SYSTEMD_NSPAWN_EXTRA_ARGS variable.

app_containers/.local/bin/run_app_container

@@ -73,7 +73,7 @@ bind_opts=""
 user_on_host=$run_as
 if [ "$CONTAINER_USE_USERNS" = true ]; then
   private_users=$(shuf -i 65536-$((2147483647 - 65536)) -n1) # Pick a random starting offset
-  bind_opts="idmap" # Note: custom mounts specified by the user are expected to incldue the idmap option as well
+  bind_opts="idmap"
   user_on_host=$((private_users + run_as))
 fi
 
@@ -90,6 +90,12 @@ for input in $(find /dev/input -type c); do
   sudo -A setfacl -m u:$user_on_host:rw- $input
 done
 
+# Bind-mounts defined by the user (possibly in the container-specific config file)
+# Format should be "src:target". target cannot be omitted
+for mount in ${CONTAINER_BIND_MOUNTS[@]}; do
+  SYSTEMD_NSPAWN_EXTRA_ARGS+=" --bind=$mount:$bind_opts"
+done
+
 sudo -A systemd-nspawn -M $CONTAINER_NAME \
     --private-users=$private_users --private-users-ownership=map \
     `# DNS (when containers do not have their own netns)` \