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

turbo.json

@@ -1,7 +1,9 @@
 {
   "$schema": "https://turbo.build/schema.json",
   "ui": "tui",
-  "globalDependencies": ["**/.env.*local"],
+  "globalDependencies": [
+    "**/.env.*local"
+  ],
   "globalEnv": [
     "DEBUG",
     "VERCEL_URL",
@@ -36,6 +38,7 @@
     },
     "start": {},
     "test:unit": {},
+    "test:integration:login": {},
     "test:acceptance": {},
     "test:e2e": {},
     "lint": {},
@@ -48,4 +51,4 @@
       "cache": false
     }
   }
-}
+}