add static user possibility

rootfs/etc/cont-init.d/04-static-user

@@ -0,0 +1,14 @@
+#!/usr/bin/with-contenv sh
+
+if [ ! -z ${STATIC_USER+x} ]; then
+    ftpasswd \
+        --sha512 \
+        --passwd \
+        --home /var/lib/ftp/data \
+        --uid ${STATIC_UID} \
+        --gid ${STATIC_GID} \
+        --name ${STATIC_USER} \
+        --shell /bin/sh \
+        --file /etc/proftpd/passwd \
+        --stdin ${STATIC_PASSWORD}
+fi