[mutter/benzea/add-podman-instructions] ci: Add note that podman needs --format docker

[Benjamin Berg <bberg src gnome org>]

commit 320defb8103eed7b2a493fac5ce19f23de0580e8
Author: Benjamin Berg <bberg redhat com>
Date:   Wed Dec 11 10:59:03 2019 +0100

    ci: Add note that podman needs --format docker
    
    Without this podman will create images that work on some–but not
    all–runners. Just adding --format docker will make the created images
    work in all cases.

 .gitlab-ci/Dockerfile | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci/Dockerfile b/.gitlab-ci/Dockerfile
index 6d2548c18..082219cfb 100644
--- a/.gitlab-ci/Dockerfile
+++ b/.gitlab-ci/Dockerfile
@@ -4,6 +4,7 @@
 #     docker build --no-cache -t registry.gitlab.gnome.org/gnome/mutter/master:v3 .
 #     docker push registry.gitlab.gnome.org/gnome/mutter/master:v3
 #
+# When using podman instead of docker, specify "--format docker"
 
 FROM fedora:31