On External SOP CI Testing

Currently we test external-sop in Codebergs Woodpecker CI against Sequoia-SOP.
For this reason, we define gradle:7.5-jdk8-jammy in .woodpecker/.build.yml, since Ubuntu Jammy already has sqop available which can therefore be installed by calling apt update && apt install --yes sqop.
At a later point we might want to upgrade the image to Lunar or Kinetic, since those distributions already have pgpainless-cli available, which we might then install as well.
Tested backends are defined in external-sop/src/main/resources/sop/external/config.json.ci which we copy to external-sop/src/main/resources/sop/external/config.json before executing the gradle test etc.