Don't copy inexistent certs folder in Dockerfile.django

[uncycler]

When building Dockerfile.django with DOCKER_BUILDKIT=1 option in docker, the build fails with:

failed to solve with frontend dockerfile.v0: failed to build LLB: lstat /var/lib/docker/tmp/buildkit-mount548069851/certs: no such file or directory

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[dsever]

@uncycler I added this option intentionally, see https://github.com/DefectDojo/Documentation/blob/master/docs/social-authentication.rst#saml-20

Idea is to enable generic way of inserting of private CA certs that are not included inside of certifi, problem is that I couldn't fine nice way to do optional copy e.g https://forums.docker.com/t/copy-only-if-file-exist/3781/2 . If you have better proposal please share it here.

[uncycler]

@dsever Maybe just adding a certs directory with a readme.txt. We could put it inside /docker so it don't clobber the root. What do you think?

[dsever]

You mean just to have some content, not to break the build? If yes than I can add readme.txt and some instructions to it.

so /docker/certs/readme.txt?

For sure we need have DOCKER_BUILDKIT=1 option when we are building images

[uncycler]

I update the PR to add a placeholder.

[madchap]

lgtm.

[valentijnscholten]

ah great, I was thinking about a placeholder also but didn't see the PR was already updated with that. approving..