Enforce environment variable configuration for Docker entrypoint

- Updated `docker-entrypoint.sh` to disallow custom command-line arguments, requiring configuration through `SCRATCHBOX_*` environment variables.
- Modified `README.md` to clarify that custom container commands are not supported and to reflect the new entrypoint behavior.
- Enhanced `main_test.go` to validate failure when required environment variables are not set.
- Added a function in `server.go` to print startup environment variables, ensuring visibility of configuration during server initialization.

cmd/scratchbox/main_test.go

@@ -139,9 +139,30 @@ func TestMainProcessEnvSuccess(t *testing.T) {
 		"SCRATCHBOX_STORAGE_DATA_DIR="+filepath.Join(tmp, "data"),
 		"SCRATCHBOX_SECURITY_ALLOWED_IPS=",
 	)
-	if out, err := cmd.CombinedOutput(); err != nil {
+	out, err := cmd.CombinedOutput()
+	if err != nil {
 		t.Fatalf("helper process failed: %v, output=%s", err, string(out))
 	}
+	if !strings.Contains(string(out), "[scratchbox] --env startup variables:") {
+		t.Fatalf("helper output missing env startup header, output=%q", string(out))
+	}
+	if !strings.Contains(string(out), "SCRATCHBOX_SERVER_LISTEN_ADDR=127.0.0.1:0") {
+		t.Fatalf("helper output missing expected env var, output=%q", string(out))
+	}
+}
+
+func TestMainProcessEnvWithoutScratchboxVarsFails(t *testing.T) {
+	t.Parallel()
+
+	cmd := exec.Command(os.Args[0], "-test.run=TestMainHelperProcess")
+	cmd.Dir = repoRoot(t)
+	cmd.Env = []string{
+		"GO_WANT_MAIN_HELPER=1",
+		"MAIN_HELPER_MODE=env-no-vars",
+	}
+	if err := cmd.Run(); err == nil {
+		t.Fatalf("expected helper process failure when --env has no SCRATCHBOX_* vars")
+	}
 }
 
 func TestServerConfigAndEnvMutuallyExclusive(t *testing.T) {
@@ -256,6 +277,10 @@ func TestMainHelperProcess(t *testing.T) {
 		os.Args = []string{"scratchbox", "server", "--env"}
 		main()
 		os.Exit(0)
+	case "env-no-vars":
+		os.Args = []string{"scratchbox", "server", "--env"}
+		main()
+		os.Exit(0)
 	case "storage-fail":
 		cfgPath := os.Getenv("MAIN_HELPER_CONFIG")
 		os.Args = []string{"scratchbox", "server", "--config", cfgPath}