Remove openjdk-8 as a dependency.

default-jre and java dependencies dependencies had been added initially
[1] because of libreoffice-java-common, which is no longer present.
Then, when the image was changed from ubuntu to alpine [2], default-jre
was replaced with openjdk-8.

If java is still a dependency for libreoffice, then it should be pulled
automatically.

[1] firstlookmedia/dangerzone-converter@9ecdb9e
[2] firstlookmedia/dangerzone-converter@650ae6e

container/Dockerfile

@@ -6,7 +6,6 @@ RUN apk -U upgrade && \
  ghostscript \
  graphicsmagick \
  libreoffice \
- openjdk8 \
  poppler-utils \
  python3 \
  py3-magic \