chore: fix login integration (#10318)

# Which Problems Are Solved

Login integration tests are not executed in the pipeline

# How the Problems Are Solved

The login integration tests are fixed and added as a pipeline workflow.
It  tests against the built login docker image.
On pipeline failures, developers are guided on how to fix them using a
dev container configured for this purpose.

# Additional Changes

- email domains are replaced by example.com. In case the tests were
accidentally run against a cloud instance, it wouldn't cause bounces.
- pnpm is upgraded, because the --filter argument doesn't work for the
install command on the old version.
- The login Dockerfile is optimized for docker image builds

# Additional Changes From Review for
https://github.com/zitadel/zitadel/pull/10305

These changes were requested from @peintnermax 

- The base dev container starts without any services besides the
database and the dev container itself
- CONTRIBUTING.md is restructured
- To reproduce pipeline checks, only the devcontainer CLI and Docker are
needed. This is described in the CONTRIBUTING.md
- The convenience npm script "generate" is added

# Additional Context

- Follow-up for PR https://github.com/zitadel/zitadel/pull/10305
- Base for https://github.com/zitadel/zitadel/issues/10277

.devcontainer/login-integration/docker-compose.yaml

@@ -0,0 +1,35 @@
+services:
+  login-integration-dev:
+    extends:
+      file: ../base/docker-compose.yaml
+      service: devcontainer
+    container_name: login-integration-dev
+    env_file: ../../apps/login/.env.test
+    environment:
+      CORE_MOCK_STUBS_URL: http://localhost:22220/v1/stubs
+      LOGIN_BASE_URL: http://localhost:3001/ui/v2/login
+      CYPRESS_CACHE_FOLDER: /workspaces/.artifacts/cypress
+    network_mode: service:mock-zitadel
+    depends_on:
+      login-integration:
+        condition: service_healthy
+
+  login-integration:
+    container_name: login-integration
+    image: "${LOGIN_TAG:-ghcr.io/zitadel/zitadel-login:latest}"
+    build:
+      context: ../..
+      dockerfile: build/login/Dockerfile
+    env_file: ../../apps/login/.env.test
+    network_mode: service:mock-zitadel
+
+  mock-zitadel:
+    container_name: mock-zitadel
+    build:
+      context: ../../apps/login/integration/core-mock
+      additional_contexts:
+        - zitadel-protos=../../proto
+    ports:
+      - 22220:22220
+      - 22222:22222
+      - 3001:3001