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/base/devcontainer.json

@@ -1,15 +1,15 @@
 {
 	"$schema": "https://raw.githubusercontent.com/devcontainers/spec/refs/heads/main/schemas/devContainer.schema.json",
-	"name": "devcontainer",
-	"dockerComposeFile": "docker-compose.yml",
+	"name": "Base: Build and Run the Components you need",
+	"dockerComposeFile": "docker-compose.yaml",
 	"service": "devcontainer",
+	"runServices": [
+		"devContainer",
+		"db"
+	],
 	"workspaceFolder": "/workspaces",
-	"features": {
-		"ghcr.io/devcontainers/features/go:1": {
-			"version": "1.24"
-		},
-		"ghcr.io/guiyomh/features/golangci-lint:0": {},
-		"ghcr.io/jungaretti/features/make:1": {}
+	"remoteEnv": {
+		"DISPLAY": ""
 	},
 	"forwardPorts": [
 		3000,
@@ -17,12 +17,13 @@
 		4200,
 		8080
 	],
-	"onCreateCommand": "pnpm install -g sass@1.64.1",
-	"customizations": {
-		"jetbrains": {
-			"settings": {
-				"com.intellij:app:HttpConfigurable.use_proxy_pac": true
-			}
-		}
+	"onCreateCommand": "pnpm install --frozen-lockfile --recursive --prefer-offline",
+	"features": {
+		"ghcr.io/devcontainers/features/go:1": {
+			"version": "1.24"
+		},
+		"ghcr.io/guiyomh/features/golangci-lint:0": {},
+		"ghcr.io/jungaretti/features/make:1": {},
+		"ghcr.io/devcontainers/features/docker-outside-of-docker": {}
 	}
-}
+}