docker: add an entrypoint

running as an arbitrary user is now possible setting the following
env vars too:

SFTPGO_PUID
SFTPGO_PGID

Fixes #217

docker/scripts/entrypoint.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+SFTPGO_PUID=${SFTPGO_PUID:-1000}
+SFTPGO_PGID=${SFTPGO_PGID:-1000}
+
+if [ "$1" = 'sftpgo' ]; then
+    if [ "$(id -u)" = '0' ]; then
+        getent passwd ${SFTPGO_PUID} > /dev/null
+	    HAS_PUID=$?
+	    getent group ${SFTPGO_PGID} > /dev/null
+	    HAS_PGID=$?
+        if [ ${HAS_PUID} -ne 0 ] || [ ${HAS_PGID} -ne 0 ]; then
+            echo `date +%Y-%m-%dT%H:%M:%S.%3N` - "entrypoint, prepare to run as uid: ${SFTPGO_PUID} gid: ${SFTPGO_PGID}"
+            if [ ${HAS_PGID} -ne 0 ];  then
+                echo `date +%Y-%m-%dT%H:%M:%S.%3N` - "entrypoint, set GID to: ${SFTPGO_PGID}"
+                groupmod -g ${SFTPGO_PGID} sftpgo
+            fi
+            if [ ${HAS_PUID} -ne 0 ]; then
+                echo `date +%Y-%m-%dT%H:%M:%S.%3N` - "entrypoint, set UID to: ${SFTPGO_PUID}"
+                usermod -u ${SFTPGO_PUID} sftpgo
+            fi
+            chown -R ${SFTPGO_PUID}:${SFTPGO_PGID} /etc/sftpgo
+            chown ${SFTPGO_PUID}:${SFTPGO_PGID} /var/lib/sftpgo /srv/sftpgo
+        fi
+        echo `date +%Y-%m-%dT%H:%M:%S.%3N` - "entrypoint, run as uid: ${SFTPGO_PUID} gid: ${SFTPGO_PGID}"
+        exec gosu ${SFTPGO_PUID}:${SFTPGO_PGID} "$@"
+    fi
+
+    exec "$@"
+fi
+
+exec "$@"