[Bug] Pulling image is now based on `push_time` which is not updated if image is identical
**Describe the bug**
In `pull_if_newer()`, we pull the image if the remote timestamp is newer than the local one. Recently, we changed this (https://github.com/vantage6/vantage6/pull/482), and based the timestamp on the push time. 

It can occur that the local and remote image are identical, but that the remote push timestamp is newer. Then, the 'newer' remote image is pulled over and over again without ever updating the timestamp. This should be achieved somehow so that no pulls are attempted.

Note that when the images are identical, the pull time is short, so this issue is not very pressing for that reason.

diff --git a/vantage6-common/vantage6/common/docker/addons.py b/vantage6-common/vantage6/common/docker/addons.py
index e1ba4ae4..4a0c670b 100644
--- a/vantage6-common/vantage6/common/docker/addons.py
+++ b/vantage6-common/vantage6/common/docker/addons.py
@@ -149,7 +149,7 @@ def inspect_remote_image_timestamp(
         log.warn("Or an image from docker hub?")
         log.warn("We'll make a final attempt when running the image to pull"
                  " it without any checks...")
-        return
+        return None, None
 
     # figure out API of the docker repo
     v1_check = requests.get(f"https://{reg}/api/health")
@@ -163,7 +163,7 @@ def inspect_remote_image_timestamp(
         log.error(f"Could not determine version of the registry! {reg}")
         log.error("Is this a Harbor registry?")
         log.error("Or is the harbor server offline?")
-        return
+        return None, None
 
     if v1:
         image = f"https://{reg}/api/repositories/{rep}/{img_}/tags/{tag}"
@@ -179,12 +179,12 @@ def inspect_remote_image_timestamp(
     # verify that we got an result
     if result.status_code == 404:
         log.warn(f"Remote image not found! {image}")
-        return
+        return None, None
 
     if result.status_code != 200:
         log.warn(f"Remote info could not be fetched! ({result.status_code}) "
                  f"{image}")
-        return
+        return None, None
 
     if v1:
         timestamp = parse(result.json().get("created"))
@@ -221,10 +221,10 @@ def inspect_local_image_timestamp(
         img = docker_client.images.get(image)
     except docker.errors.ImageNotFound:
         log.debug(f"Local image does not exist! {image}")
-        return None
+        return None, None
     except docker.errors.APIError:
         log.debug(f"Local info not available! {image}")
-        return None
+        return None, None
 
     try:
         digest = img.attrs.get("RepoDigests")[0].split("@")[1]
['vantage6-common/vantage6/common/docker/addons.py']