chore(deps): update ghcr.io/jeboehm/fetchmailmgr docker tag to v0.3.15 (#471)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

docker-compose.yml

@@ -108,7 +108,7 @@ services:
   # and deliver them to the local mail server. Can be removed
   # if you don't need it.
   fetchmail:
-    image: ghcr.io/jeboehm/fetchmailmgr:0.3.12
+    image: ghcr.io/jeboehm/fetchmailmgr:0.3.15@sha256:09bf40544e4a24e43581358416f4d29d980bd3c820be571a7df5b6642af2a5d9
     restart: on-failure:5
     command:
       - --interval