diff --git a/.docker/Dockerfile b/.docker/Dockerfile
index c3b8a10ff4c72435e0a07a7f6fcb2f7837bd989d..1a674a160f4f665419edd48466c47e65c3a5836c 100644
--- a/.docker/Dockerfile
+++ b/.docker/Dockerfile
@@ -12,5 +12,6 @@ COPY doc/requirements.txt requirements.txt
 RUN pip3 install -r requirements.txt
 
 RUN echo "deb ftp.de.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/buster-backports.list
+RUN apt-get update
 RUN apt-get install -y cmake=3.16.3-3~bpo10+1