Fix builds on older Docker

The Fedora image doesn't have an entrypoint defined, and while bleeding
edge Docker implements a fallback, older versions don't.

.gitlab-ci.yml

@@ -4,7 +4,9 @@ stages:
   - report
 
 default:
-  image: registry.gitlab.com/kicad/kicad-ci/source_containers/master/fedora:34
+  image:
+    name: registry.gitlab.com/kicad/kicad-ci/source_containers/master/fedora:34
+    entrypoint: ["/bin/sh", "-c"]
 
 
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