[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 70d2e295..e1ba4ae4 100644
--- a/vantage6-common/vantage6/common/docker/addons.py
+++ b/vantage6-common/vantage6/common/docker/addons.py
@@ -116,7 +116,7 @@ def registry_basic_auth_header(
 def inspect_remote_image_timestamp(
     docker_client: DockerClient, image: str,
     log: logging.Logger | ClickLogger = ClickLogger
-) -> datetime | None:
+) -> tuple[datetime, str] | None:
     """
     Obtain creation timestamp object from remote image.
 
@@ -131,9 +131,9 @@ def inspect_remote_image_timestamp(
 
     Returns
     -------
-    datetime | None
-        Timestamp containing the creation date and time of the image, or None
-        if the remote image could not be found.
+    tuple[datetime, str] | None
+        Timestamp containing the creation date of the image and its digest, or
+        None if the remote image could not be found.
     """
     # check if a tag has been profided
 
@@ -188,15 +188,17 @@ def inspect_remote_image_timestamp(
 
     if v1:
         timestamp = parse(result.json().get("created"))
+        digest = None
     else:
         timestamp = parse(result.json().get("push_time"))
-    return timestamp
+        digest = result.json().get("digest")
+    return timestamp, digest
 
 
 def inspect_local_image_timestamp(
     docker_client: DockerClient, image: str,
     log: logging.Logger | ClickLogger = ClickLogger
-) -> datetime | None:
+) -> tuple[datetime, str] | None:
     """
     Obtain creation timestamp object from local image.
 
@@ -211,14 +213,10 @@ def inspect_local_image_timestamp(
 
     Returns
     -------
-    datetime | None
-        Timestamp containing the creation date and time of the local image. If
-        the image does not exist, None is returned.
+    tuple[datetime, str] | None
+        Timestamp containing the creation date and time of the local image and
+        the image digest. If the image does not exist, None is returned.
     """
-    # p = re.split(r"[/:]", image)
-    # if len(p) == 4:
-    #     image = f"{p[0]}/{p[1]}/{p[2]}:{p[3]}"
-
     try:
         img = docker_client.images.get(image)
     except docker.errors.ImageNotFound:
@@ -228,9 +226,13 @@ def inspect_local_image_timestamp(
         log.debug(f"Local info not available! {image}")
         return None
 
+    try:
+        digest = img.attrs.get("RepoDigests")[0].split("@")[1]
+    except Exception:
+        digest = None
     timestamp = img.attrs.get("Created")
     timestamp = parse(timestamp)
-    return timestamp
+    return timestamp, digest
 
 
 def pull_if_newer(
@@ -254,23 +256,29 @@ def pull_if_newer(
     docker.errors.APIError
         If the image cannot be pulled
     """
-    local_ = inspect_local_image_timestamp(docker_client, image, log=log)
-    remote_ = inspect_remote_image_timestamp(docker_client, image, log=log)
+    local_time, local_digest = inspect_local_image_timestamp(
+        docker_client, image, log=log
+    )
+    remote_time, remote_digest = inspect_remote_image_timestamp(
+        docker_client, image, log=log
+    )
     pull = False
-    if local_ and remote_:
-        if remote_ > local_:
+    if local_time and remote_time:
+        if remote_digest == local_digest:
+            log.debug(f"Local image is up-to-date: {image}")
+        elif remote_time > local_time:
             log.debug(f"Remote image is newer: {image}")
             pull = True
-        elif remote_ == local_:
+        elif remote_time == local_time:
             log.debug(f"Local image is up-to-date: {image}")
-        elif remote_ < local_:
+        elif remote_time < local_time:
             log.warn(f"Local image is newer! Are you testing? {image}")
-    elif local_:
+    elif local_time:
         log.warn(f"Only a local image has been found! {image}")
-    elif remote_:
+    elif remote_time:
         log.debug("No local image found, pulling from remote!")
         pull = True
-    elif not local_ and not remote_:
+    elif not local_time and not remote_time:
         log.warn(f"Cannot locate image {image} locally or remotely. Will try "
                  "to pull it from Docker Hub...")
         # we will try to pull it from the docker hub
['vantage6-common/vantage6/common/docker/addons.py']