apply advices of @yosifkit and install gnupg in images

library/convertigo

@@ -1,6 +1,6 @@
 Maintainers: Nicolas Albert <[email protected]> (@nicolas-albert), Olivier Picciotto <[email protected]> (@opicciotto)
 GitRepo: https://github.com/convertigo/docker
-GitCommit: ade9f4f55e8cefa91200b9b8e58e5b48dd1ccd16
+GitCommit: 7a231c19a6f6a8f2d9899c6f064d2ab3933e61f6
 
 Tags: 7.5.0, 7.5, latest
 Architectures: amd64, arm32v7, arm64v8, i386