Build fix for Docker build

he current Ubuntu 16.04 based build fails:

Step 7/13 : RUN cget install -DCMAKE_C_FLAGS="-fPIC" -DCMAKE_CXX_FLAGS="-fPIC" -f requirements.txt     && mkdir -p ${SRC_DIR}/build
 ---> Running in 0608c8b7e156
Traceback (most recent call last):
  File "/usr/local/bin/cget", line 7, in <module>
    from cget.cli import cli
  File "/usr/local/lib/python2.7/dist-packages/cget/cli.py", line 1, in <module>
    import click, os, functools, sys
ImportError: No module named click
The command '/bin/sh -c cget install -DCMAKE_C_FLAGS="-fPIC" -DCMAKE_CXX_FLAGS="-fPIC" -f requirements.txt     && mkdir -p ${SRC_DIR}/build' returned a non-zero code: 1

Updating to 18.04 seems to succeed

statgen / EPACTS

Build fix for Docker build #34