Fix Dockerfile

[fixdocker]

Hello, I represent a research group investigating build failures in
Dockerfiles. We executed a Dockerfile in your project and found that
it is not building. We are sending this Pull Request with a suggested
repair. The repair operation is based on the following URL:

• E: Package 'libpng12-dev' has no installation candidate docker-library/php#662 (comment)

Please note that we are using an anonymous GitHub account because of
double-blind restrictions in the submission process of conferences.

[fixdocker]

Hey @JensErat @rwky , can you check this PR?

[rwky]

I've not used selfoss in some time but looking at the repo it'd be better to upgraded to a supported PHP version (ideally 7.4)

[fixdocker]

@rwky, we have changed the base image to php: 7.4-apache, and after that we also noticed problems with the docker-php-ext configuration so we changed from --with-jpeg-dir= to --with-jpeg=[1] and remove mbstring[2] , and this fixed it. Can we submit a new pull request with these changes?

[1] - docker-library/php#931 (comment)
[2] - https://stackoverflow.com/a/59253249