Use "exec python3" in docker-entrypoint.sh

To reduce the processes in metube container

docker-entrypoint.sh

@@ -12,8 +12,8 @@ if [ `id -u` -eq 0 ] && [ `id -g` -eq 0 ]; then
     echo "Changing ownership of download and state directories to ${UID}:${GID}"
     chown -R "${UID}":"${GID}" /app "${DOWNLOAD_DIR}" "${STATE_DIR}" "${TEMP_DIR}"
     echo "Running MeTube as user ${UID}:${GID}"
-    su-exec "${UID}":"${GID}" python3 app/main.py
+    exec su-exec "${UID}":"${GID}" python3 app/main.py
 else
     echo "User set by docker; running MeTube as `id -u`:`id -g`"
-    python3 app/main.py
+    exec python3 app/main.py
 fi