Make the keyserver lookup more robust

See: nodejs/docker-node#380

Author: Sam Davies

Dockerfile

@@ -29,7 +29,9 @@ RUN set -ex \
     DD8F2338BAE7501E3DD5AC78C273792F7D83545D \
     A48C2BEE680E841632CD4E44F07496B3EB3C1762 \
   ; do \
-    gpg --keyserver pool.sks-keyservers.net --recv-keys "$key" ; \
+      gpg --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys "$key" || \
+      gpg --keyserver hkp://ipv4.pool.sks-keyservers.net --recv-keys "$key" || \
+      gpg --keyserver hkp://pgp.mit.edu:80 --recv-keys "$key" ; \
 done
 
 # Node