[WIP] Enable ubuntu-ports for non-x86 architectures

Author: infinity0

ci/docker/aarch64-unknown-linux-gnu/Dockerfile

@@ -1,5 +1,8 @@
 FROM ubuntu:18.04
 RUN dpkg --add-architecture arm64 && \
+    sed -e "\,security.ubuntu.com/ubuntu,p;s,security.ubuntu.com/ubuntu,ports.ubuntu.com/ubuntu-ports,g" \
+        -e "\,archive.ubuntu.com/ubuntu,p;s,archive.ubuntu.com/ubuntu,ports.ubuntu.com/ubuntu-ports,g" \
+        -i /etc/apt/sources.list /etc/apt/sources.list.d/* && \
     apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \