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

e2e/package.json

@@ -17,7 +17,7 @@
     "clean": "rm -rf .turbo node_modules"
   },
   "private": true,
-  "dependencies": {
+  "devDependencies": {
     "@types/pg": "^8.11.6",
     "cypress-wait-until": "^3.0.2",
     "jsonwebtoken": "^9.0.2",
@@ -26,10 +26,8 @@
     "prettier": "^3.3.3",
     "typescript": "^5.5.4",
     "uuid": "^10.0.0",
-    "wait-on": "^7.2.0"
-  },
-  "devDependencies": {
+    "wait-on": "^7.2.0",
     "@types/node": "^22.3.0",
     "cypress": "^14.5.3"
   }
-}
+}