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

.github/workflows/build.yml

@@ -102,6 +102,12 @@ jobs:
       login_build_image_name: "ghcr.io/zitadel/zitadel-login-build"
       node_version: "20"
 
+  login-integration-test:
+    uses: ./.github/workflows/login-integration-test.yml
+    needs: [login-container]
+    with:
+      login_build_image: ${{ needs.login-container.outputs.login_build_image }}
+
   e2e:
     uses: ./.github/workflows/e2e.yml
     needs: [compile]
@@ -121,6 +127,7 @@ jobs:
         lint,
         container,
         login-container,
+        login-integration-test,
         e2e,
       ]
     if: ${{ github.event_name == 'workflow_dispatch' }}