Undocumented dependencies of v4 while running in a container

[Zharktas]

We updated our workflows to run v4 of the action few weeks ago and where baffled why the uploads were failing in one of the jobs but not all of them. Apparently our container did not have the required dependencies installed and therefore the upload failed. I'm fine with adding the required dependencies gpg and gpg-agent to our testing containers but would appreciate that it would documented somewhere instead of scouring closed issues for reasons the upload fails. Additionally had to define that the container is alpine based which i found from #1262 (comment)

Changes required to our alpine based containers can be found here vrk-kpa/restricteddata#253