Use a fast Alpine package mirror

Without forcing a specific Alpine package mirror, it seems to use
the very slow default one.  This means building the docker image
can take 2+ hours (as happened yesterday).

With this change in place, building the docker image takes around
5-6 minutes.

docker/Dockerfile

@@ -21,6 +21,11 @@ RUN addgroup -S rabbitmq && \
     adduser -S -D -h /var/lib/rabbitmq -s /sbin/nologin -g "Linux User,,," -G rabbitmq rabbitmq && \
     chown -Rh rabbitmq: /opt/rabbitmq /var/log/rabbitmq
 
+# Use a fast Australian mirror for the Alpine package repositories
+# Without doing this, building the image can take 2+ hours. :(
+RUN echo "https://mirror.aarnet.edu.au/pub/alpine/v3.18/main" > /etc/apk/repositories && \
+    echo "https://mirror.aarnet.edu.au/pub/alpine/v3.18/community" >> /etc/apk/repositories
+
 # Install Git, Go, Memcached, Minio, and PostgreSQL
 RUN apk update && \
     apk upgrade && \