add Dockerfile CVE check, update vulnerability scanning to use trivy action

[tomkralidis]

Overview

add Dockerfile CVE check, update vulnerability scanning to use trivy action

Related Issue / Discussion

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(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• [x] I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
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[kalxas]

Testing only the docker image for vulnerabilities could give the impression that all deployment methods are tested as well.

In order to pass the tests here, we would also need to change the base docker image and put effort to catch up with OS security patches.

[tomkralidis]

PR updated. Note that upgrading Dockerfile to python:3.10-slim-buster and adding apt upgrade passes the trivy image scan.