plugincontainer: fix image build

[johanbrandhorst]

Lock the image version used in test, and switch to available commands

[johanbrandhorst]

This looks like it's fixed the adduser error, but now we've got another error:

{"@level":"error","@message":"plugin init error","@timestamp":"2024-07-25T00:13:13.289333Z","error":"open /tmp/go-plugin-container/plugin186839857: permission denied"}

https://github.com/hashicorp/go-secure-stdlib/actions/runs/10085840459/job/27887369639?pr=129

I tried chmod-ing the binary to 0755 but it seems it's still happening, so not sure where to go from here. I don't really have the time to dedicate to fixing this, so I would appreciate help from the community who are more interested in having this fixed soon.