diff --git a/core/Dockerfile b/core/Dockerfile
index 6165979..43e5c94 100644
--- a/core/Dockerfile
+++ b/core/Dockerfile
@@ -7,17 +7,18 @@ COPY docker-entrypoint.sh /usr/local/bin/
 
 #Install all required packages, add user for executing mailman and set execution rights for startup script
 RUN apk update \
-	&& apk add --virtual build-deps gcc python3-dev musl-dev postgresql-dev \
-	   libffi-dev \
+    && apk add --virtual build-deps gcc python3-dev musl-dev postgresql-dev \
+       libffi-dev \
   # psutil needs linux-headers to compile on musl c library.
-	&& apk add --no-cache bash su-exec postgresql-client mysql-client curl python3 py3-pip linux-headers \
-	&& python3 -m pip install -U pip setuptools wheel \
+    && apk add --no-cache bash su-exec postgresql-client mysql-client curl python3 py3-pip linux-headers \
+    && python3 -m pip install -U pip setuptools wheel \
         && python3 -m pip install psycopg2 \
                    gunicorn==19.9.0 \
                    mailman==3.3.1 \
                    mailman-hyperkitty==1.1.0 \
                    pymysql \
-		   'dnspython<2.0' \
+                   'dnspython<2.0' \
+                   'flufl.lock<5.0' \
     && apk del build-deps \
     && adduser -S mailman