docker: use Canonical's Azure apt mirror for arm64

ports.ubuntu.com failed three consecutive CI runs; the Azure mirror is
reachable from GitHub runners and the public internet alike.

Author: CMGS

docker/Dockerfile.builder

@@ -3,7 +3,10 @@ FROM ubuntu:22.04
 ARG FFMPEG_COMMIT=998de74adf861c26df557e220996faa959419549
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN echo 'Acquire::Retries "5";' > /etc/apt/apt.conf.d/80-retries \
+# ports.ubuntu.com (arm64) flakes regularly; Canonical's Azure mirror is
+# reachable from GitHub runners and the public internet alike.
+RUN sed -i 's|http://ports.ubuntu.com|http://azure.ports.ubuntu.com|g' /etc/apt/sources.list \
+    && echo 'Acquire::Retries "5";' > /etc/apt/apt.conf.d/80-retries \
     && apt-get update \
     && apt-get install -y --no-install-recommends \
        build-essential ca-certificates cmake curl file git nasm pkg-config \

docker/Dockerfile.runtime

@@ -3,7 +3,8 @@ FROM ubuntu:22.04
 ARG TARGETARCH
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN echo 'Acquire::Retries "5";' > /etc/apt/apt.conf.d/80-retries \
+RUN sed -i 's|http://ports.ubuntu.com|http://azure.ports.ubuntu.com|g' /etc/apt/sources.list \
+    && echo 'Acquire::Retries "5";' > /etc/apt/apt.conf.d/80-retries \
     && apt-get update \
     && apt-get install -y --no-install-recommends libjpeg-turbo8 zlib1g \
     && rm -rf /var/lib/apt/lists/*