Alpine: Make backtraces much better

Current backtraces from cores produced by system tests miss a lot
information on Alpine Linux. Adding the 'musl-dbg' package makes them on
par with the rest of C libraries. Addition size is about 3 MB.

docker/bind9/alpine-template/Dockerfile

@@ -23,6 +23,7 @@ RUN apk add \
 	libxslt \
 	lmdb-dev \
 	make \
+	musl-dbg \
 	openssl-dev \
 	perl \
 	perl-digest-hmac \