docker image tweaking

3 jobs for master in 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #179909072
build

00:00:39

 
  Test
skipped #179909073
test
 
  Deploy
skipped #179909075
deploy
 
Name Stage Failure
failed
build Build
    unable to execute 'cc': No such file or directory
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Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?
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error: command 'gcc' failed with exit status 1

----------------------------------------
Command "/usr/local/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-b6eelzvn/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-m1hi_j_v/install-record.txt --single-version-externally-managed --compile --home=/tmp/pip-target-tm63g6hz" failed with error code 1 in /tmp/pip-install-b6eelzvn/lxml/
ERROR: Job failed: exit code 1