Add pgp.mit.edu fallback in Dockerfile

Signed-off-by: Tibor Vass <tibor@docker.com>

Dockerfile

@@ -26,11 +26,13 @@
 FROM ubuntu:trusty
 
 # add zfs ppa
-RUN apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys E871F18B51E0147C77796AC81196BA81F6B0FC61
+RUN apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys E871F18B51E0147C77796AC81196BA81F6B0FC61 \
+	|| apt-key adv --keyserver hkp://pgp.mit.edu:80 --recv-keys E871F18B51E0147C77796AC81196BA81F6B0FC61
 RUN echo deb http://ppa.launchpad.net/zfs-native/stable/ubuntu trusty main > /etc/apt/sources.list.d/zfs.list
 
 # add llvm repo
-RUN apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 6084F3CF814B57C1CF12EFD515CF4D18AF4F7421
+RUN apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 6084F3CF814B57C1CF12EFD515CF4D18AF4F7421 \
+	|| apt-key adv --keyserver hkp://pgp.mit.edu:80 --recv-keys 6084F3CF814B57C1CF12EFD515CF4D18AF4F7421
 RUN echo deb http://llvm.org/apt/trusty/ llvm-toolchain-trusty main > /etc/apt/sources.list.d/llvm.list
 
 # Packaged dependencies