Pin and update test container image references regularly

Assisted-by: Copilot v1.0.59 (Claude Sonnet 4.6)
Signed-off-by: Pablo Zmdl <pablo@nextcloud.com>

php/tests/Containers/composer/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/library/composer:latest
+FROM docker.io/library/composer:2@sha256:7725eb4545c438629ae8bde3ef0bb9a5038ef566126ad878442a69007242d267
 
 RUN pecl bundle -d /usr/src/php/ext apcu \
  && docker-php-ext-install apcu

php/tests/compose.yaml

@@ -59,21 +59,21 @@ services:
       - local-code
 
   desec-mock:
-    image: docker.io/library/node:latest
+    image: docker.io/library/node:26@sha256:b46a10d964ad15136ebdf9012142131481caa0697d7a4d4eafe4bbabd818f876
     volumes:
       - ..:/app/php
     working_dir: /app
     command: node php/tests/desec-mock.mjs 8090 2>&1
           
   npm-installer:
-    image: docker.io/library/node:latest
+    image: docker.io/library/node:26@sha256:b46a10d964ad15136ebdf9012142131481caa0697d7a4d4eafe4bbabd818f876
     volumes:
       - ..:/app
     working_dir: /app/tests
     command: npm ci --omit=optional
 
   test-runner-base:
-    image: mcr.microsoft.com/playwright:v1.56.1
+    image: mcr.microsoft.com/playwright:v1.56.1@sha256:f1e7e01021efd65dd1a2c56064be399f3e4de00fd021ac561325f2bfbb2b837a
     volumes:
       - ..:/app
     working_dir: /app/tests