Make run-tests compatible with Podman (#3961)

There is no need to filter for references in such a complicated way.
docker images already works if you just pass the image and tag as
an argument.

This caused problems if one was using podman with its docker-compatible
interface.
Previously podman would return the following error:
Error: cannot specify an image and a filter(s)

With this new method podman does not return an error anymore, causing
the image to not be redownloaded every time and it still works with
normal Docker.

run-tests

@@ -170,7 +170,7 @@ fi
 find test -name '*.swp' -delete
 
 # Check if docker un image is available locally
-has_image=$(docker images ls --filter reference="${image}:${image_tag}" --quiet | wc -l)
+has_image=$(docker images --quiet "${image}:${image_tag}" | wc -l)
 
 if [ "$has_image" -eq 0 ]
 then