don't pin digests anymore

The benefit of digest pinning isn't that great for this project really
and it comes at the cost of more issues and additional renovate noise,
so just don't anymore.
Adjust renovate config accordingly as well.

resources/chaosknoten/pretalx/docker_compose/compose.yaml.j2

@@ -3,7 +3,7 @@
 
 services:
   database:
-    image: docker.io/library/postgres:15-alpine@sha256:64583b3cb4f2010277bdd9749456de78e5c36f8956466ba14b0b96922e510950
+    image: docker.io/library/postgres:15-alpine
     environment:
       - "POSTGRES_USER=pretalx"
       - "POSTGRES_PASSWORD={{ secret__pretalx_db_password }}"
@@ -15,7 +15,7 @@ services:
       - pretalx_net
 
   redis:
-    image: docker.io/library/redis:8.2.2@sha256:4521b581dbddea6e7d81f8fe95ede93f5648aaa66a9dacd581611bf6fe7527bd
+    image: docker.io/library/redis:8.2.2
     restart: unless-stopped
     volumes:
       - redis:/data
@@ -23,7 +23,7 @@ services:
       - pretalx_net
 
   static:
-    image: docker.io/library/nginx:1.29.3@sha256:f547e3d0d5d02f7009737b284abc87d808e4252b42dceea361811e9fc606287f
+    image: docker.io/library/nginx:1.29.3
     restart: unless-stopped
     volumes:
       - public:/usr/share/nginx/html
@@ -33,7 +33,7 @@ services:
       - pretalx_net
 
   pretalx:
-    image: docker.io/pretalx/standalone:v2025.1.0@sha256:fb2d15f11bcae8bb15430084ed81a150cfdf7c79705450583b51e352ba486e8e
+    image: docker.io/pretalx/standalone:v2025.1.0
     entrypoint: gunicorn
     command:
       - "pretalx.wsgi"
@@ -78,7 +78,7 @@ services:
       - pretalx_net
 
   celery:
-    image: docker.io/pretalx/standalone:v2025.1.0@sha256:fb2d15f11bcae8bb15430084ed81a150cfdf7c79705450583b51e352ba486e8e
+    image: docker.io/pretalx/standalone:v2025.1.0
     command:
       - taskworker
     restart: unless-stopped