Add basic CI to builds garfieldpp

Using root's own docker images as a base, I add automated build tests for garfieldpp

This will be a vital tool to help the garfieldpp maintainers to evaluate future merge request

C++11 builds include

• centos 7 (using view 97 as per build instructions at https://garfieldpp.web.cern.ch/garfieldpp/getting-started/)
• centos 7 (root 6.22.02) - currently broken, see below
• ubuntu 20.04 (tracks latest root version, at time of writing is 6.22.06)

C++14 builds include:

• fedora 32 (root 6.22.02)
• fedora 33 (root 6.22.06)

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The 'build-centos7' currently fails, however I propose a fix in merge request 170: !170 (merged) I demonstrate merge request 170 works in my own branch here https://gitlab.cern.ch/jmckenna/garfieldpp/-/pipelines/2314880 (it is simply this merge request with merge request 170)