Try multiple keyservers

Fixes #64 with the fix from @andyneff in tianon/gosu#53

Dockerfile

@@ -36,7 +36,13 @@ RUN set -ex \
     && wget -O /usr/local/bin/gosu.asc "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$dpkgArch.asc" \
 # verify the signature
     && export GNUPGHOME="$(mktemp -d)" \
-    && gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 \
+    && for server in $(shuf -e ha.pool.sks-keyservers.net \
+                            hkp://p80.pool.sks-keyservers.net:80 \
+                            keyserver.ubuntu.com \
+                            hkp://keyserver.ubuntu.com:80 \
+                            pgp.mit.edu) ; do \
+        gpg --keyserver "$server" --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 && break || : ; \
+    done \
     && gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu \
     && rm -rf "$GNUPGHOME" /usr/local/bin/gosu.asc \
     && chmod +x /usr/local/bin/gosu \