Dockerfile-release: add nsswitch.conf into image

The file '/etc/nsswitch.conf' is created in order to
take in account '/etc/hosts' entries while resolving
domain names.

Dockerfile-release

@@ -5,6 +5,12 @@ ADD etcdctl /usr/local/bin/
 RUN mkdir -p /var/etcd/
 RUN mkdir -p /var/lib/etcd/
 
+# Alpine Linux doesn't use pam, which means that there is no /etc/nsswitch.conf,
+# but Golang relies on /etc/nsswitch.conf to check the order of DNS resolving
+# (see https://github.com/golang/go/commit/9dee7771f561cf6aee081c0af6658cc81fac3918)
+# To fix this we just create /etc/nsswitch.conf and add the following line:
+RUN echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' >> /etc/nsswitch.conf
+
 EXPOSE 2379 2380
 
 # Define default command.