Add a dnsfix service

Test to see if we can reach the DNS server which we get via
DHCP. If not fall back to Google public DNS.

Some users on OSX seem to get this situation with xhyve,
see https://github.com/docker/pinata/issues/551

Signed-off-by: Justin Cormack <justin.cormack@unikernel.com>

alpine/Dockerfile

@@ -36,6 +36,7 @@ COPY packages/automount/etc /etc/
 COPY packages/9pinit/etc /etc/
 COPY packages/ntp15m/etc /etc/
 COPY packages/binfmt_misc/etc /etc/
+COPY packages/dnsfix/etc /etc/
 
 RUN \
   rc-update add swap boot && \
@@ -68,6 +69,7 @@ RUN \
   rc-update add automount boot && \
   rc-update add diagnostics default && \
   rc-update add binfmt_misc sysinit && \
+  rc-update add dnsfix boot && \
   ln -s /bin/busybox /init
 
 CMD ["/bin/sh"]

alpine/packages/dnsfix/etc/init.d/dnsfix

@@ -0,0 +1,18 @@
+#!/sbin/openrc-run
+
+# A few users have reported not being able to access the nameserver on OSX
+# If a lookup fails, fall back to Google public nameserver
+
+depend()
+{
+	after networking
+}
+
+start()
+{
+	ebegin "Testing DNS resolution"
+
+	dig www.docker.com 2>&1 > /dev/null || printf "nameserver 8.8.8.8\nnameserver 8.8.4.4\n" > /etc/resolv.conf
+
+	eend $? "DNS fix failed"
+}