[MXNET-703] Install CUDA 10 compatible cmake

This works around a CUDA 10 cmake issue documented here:
clab/dynet#1457

This fix is temporary; once an updated cmake package is published to Ubuntu's
package repo it may be reverted.

ci/docker/install/ubuntu_core.sh

@@ -48,3 +48,12 @@ if [[ $(lsb_release -r | grep 14.04) ]]; then
 else
     apt-get install -y cmake
 fi
+# Note: we specify an exact cmake version to work around a cmake 3.10 CUDA 10 issue.
+# Reference: https://github.com/clab/dynet/issues/1457
+# Testing builds without custom cmake workign around issue
+# mkdir /opt/cmake && cd /opt/cmake
+# wget -nv https://cmake.org/files/v3.12/cmake-3.12.4-Linux-x86_64.sh
+# sh cmake-3.12.4-Linux-x86_64.sh --prefix=/opt/cmake --skip-license
+# ln -s /opt/cmake/bin/cmake /usr/local/bin/cmake
+# rm cmake-3.12.4-Linux-x86_64.sh
+# cmake --version