Re-evaluate container dependencies to mitigate potential CVEs

[deeplow]

We can improve the security on the same level as #222 by reducing the software we ship in the container and replace vulerability-prone software.

This includes a re-evaluation of the software in container/dangerzone.py and container/Dockerfile.

Notable examples are:

• removing sudo dependency
• GraphicsMagick - used in the first (more exposed container) as well as in the second one. The Qubes
• Libreoffice - perhaps shipping a headless / more stripped down version
• supply chain security: making sure we only download packages from verified sources (in particular PDFtk)

[apyrgio]

I'd add removing sudo in the above list. Other than that, I fully agree.

[deeplow]

Removing openjdk-8 dependecy

default-jre and java dependencies dependencies had been added initially because of libreoffice-java-common, which is no longer present.

Then, when the image was changed from ubuntu to alpine, default-jre was replaced with openjdk-8.

If java is still a dependency for libreoffice, then it should be pulled automatically. There should be no need to state it explicitly. And tests pass.