diff --git a/Dockerfile b/Dockerfile
index f535d91..31d6b51 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -19,7 +19,10 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 
 WORKDIR /app
 
- RUN apt-get update \
+# Use HTTPS mirrors so apt works even if HTTP (port 80) is blocked in the network.
+RUN sed -i 's|http://deb.debian.org|https://deb.debian.org|g' /etc/apt/sources.list \
+    && sed -i 's|http://security.debian.org|https://security.debian.org|g' /etc/apt/sources.list \
+    && apt-get update \
     && apt-get install -y --no-install-recommends libgomp1 \
     && rm -rf /var/lib/apt/lists/*