ci: pull service images from the mirror.gcr.io cache

ECR Public refused the PostgreSQL, MySQL, and Redis image pulls with
"toomanyrequests: Data limit exceeded", so the database jobs failed
before a test ran. mirror.gcr.io serves the same Docker Hub library
images. The JavaScript repository moved to it in
cardmagic/solid-objects-js#79.

Author: cardmagic

.github/workflows/ci.yml

@@ -66,7 +66,7 @@ jobs:
     timeout-minutes: 15
     services:
       redis:
-        image: public.ecr.aws/docker/library/redis:7
+        image: mirror.gcr.io/library/redis:7
         ports:
           - 6379:6379
         options: >-
@@ -87,7 +87,7 @@ jobs:
     timeout-minutes: 15
     services:
       postgres:
-        image: public.ecr.aws/docker/library/postgres:18
+        image: mirror.gcr.io/library/postgres:18
         env:
           POSTGRES_USER: solid_objects
           POSTGRES_PASSWORD: solid_objects
@@ -121,7 +121,7 @@ jobs:
         client: [ mysql2, trilogy ]
     services:
       mysql:
-        image: public.ecr.aws/docker/library/mysql:8.4
+        image: mirror.gcr.io/library/mysql:8.4
         env:
           MYSQL_USER: solid_objects
           MYSQL_PASSWORD: solid_objects