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/turbo-lint-unit/devcontainer.json

@@ -1,18 +1,20 @@
 {
 	"$schema": "https://raw.githubusercontent.com/devcontainers/spec/refs/heads/main/schemas/devContainer.schema.json",
-	"name": "turbo-lint-unit",
+	"name": "Turbo Lint and Unit Tests",
 	"dockerComposeFile": [
-		"../base/docker-compose.yml"
+		"../base/docker-compose.yaml"
 	],
 	"service": "devcontainer",
-	"runServices": ["devcontainer"],
+	"runServices": [
+		"devcontainer"
+	],
 	"workspaceFolder": "/workspaces",
-	"postStartCommand": "pnpm install --frozen-lockfile --recursive && pnpm turbo lint test:unit",
-	"customizations": {
-		"jetbrains": {
-			"settings": {
-				"com.intellij:app:HttpConfigurable.use_proxy_pac": true
-			}
-		}
-	}
-}
+	"forwardPorts": [
+		3001
+	],
+	"remoteEnv": {
+		"FAIL_COMMANDS_ON_ERRORS": "${localEnv:FAIL_COMMANDS_ON_ERRORS}"
+	},
+	"updateContentCommand": "/commands/turbo-lint-unit.update-content.sh",
+	"postAttachCommand": "/commands/turbo-lint-unit.post-attach.sh"
+}