diff --git a/docker/Dockerfile-optimised b/docker/Dockerfile-optimised
index e3af87b25676..6cce3ed3b7fd 100644
--- a/docker/Dockerfile-optimised
+++ b/docker/Dockerfile-optimised
@@ -21,7 +21,7 @@ FROM matrixdotorg/optimised-python:${PYTHON_VERSION}-optimized-lto-${MARCH} as b
 
 # install the OS build deps
 
-RUN apt-get install -y build-essential libpq5-dev libffi-dev libexpat1-dev libxml2-dev libxslt-dev
+RUN apt-get update && apt-get install -y build-essential libpq5-dev libffi-dev libexpat1-dev libxml2-dev libxslt-dev
 
 # build things which have slow build steps, before we copy synapse, so that
 # the layer can be cached.
@@ -53,7 +53,7 @@ RUN chown -R root:root /usr/local /install
 FROM matrixdotorg/optimised-python:${PYTHON_VERSION}-optimized-lto-${MARCH}
 
 # xmlsec is required for saml support
-RUN apt install libpq5 xmlsec libxslt1
+RUN apt-get update && apt-get install -y libpq5 xmlsec libxslt1
 
 COPY --from=builder /install /usr/local
 COPY ./docker/start.py /start.py