text: Title: Dockerfile not building image on Docker version 18.03.1-ce
  Body: Please Describe the issue or question and share your OS and Python version.
  _________________
  **OS**: `ubuntu`
  
  **OS Version**: `16.04`
  
  **Python Version**: `N/A`, using `Docker version 18.03.1-ce, build 9ee9f40`
  
  The Dockerfile fails at building the image with the following error:
  
  ```
      running build_ext
      building 'lxml.etree' extension
      creating build/temp.linux-x86_64-2.7
      creating build/temp.linux-x86_64-2.7/src
      creating build/temp.linux-x86_64-2.7/src/lxml
      gcc -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -DTHREAD_STACK_SIZE=0x100000 -fPIC -DCYTHON_CLINE_IN_TRACEBACK=0 -Isrc -Isrc/lxml/includes -I/usr/local/include/python2.7 -c src/lxml/etree.c -o build/temp.linux-x86_64-2.7/src/lxml/etree.o -w
      In file included from src/lxml/etree.c:660:0:
      src/lxml/includes/etree_defs.h:14:31: fatal error: libxml/xmlversion.h: No such file or directory
       #include "libxml/xmlversion.h"
                                     ^
      compilation terminated.
      Compile failed: command 'gcc' failed with exit status 1
      creating tmp
      cc -I/usr/include/libxml2 -c /tmp/xmlXPathInitbVgtAC.c -o tmp/xmlXPathInitbVgtAC.o
      /tmp/xmlXPathInitbVgtAC.c:1:26: fatal error: libxml/xpath.h: No such file or directory
       #include "libxml/xpath.h"
                                ^
      compilation terminated.
      *********************************************************************************
      Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?
      *********************************************************************************
      error: command 'gcc' failed with exit status 1
      
      ----------------------------------------
  Command "/usr/local/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-WQQF7N/lxml/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-is3w6W/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-install-WQQF7N/lxml/
  The command '/bin/sh -c pip install --no-cache-dir -r requirements.txt' returned a non-zero code: 1
  ```
  