fix(e2e): use host network for Docker image builds in DinD environments

Docker-in-Docker containers cannot reach the internet via the default
bridge network, causing 'apk add' in the Dockerfile to hang indefinitely
and e2e tests to timeout after 10 minutes.

Add BuildOptionsModifier with NetworkMode=host to all FromDockerfile
definitions so the build step uses the host's network stack. This has
no negative impact on non-DinD environments where bridge networking
already works.

Also move GostBinPath flag and init() from main_test.go to utils.go
(non-test file) so the symbol is accessible across the package.

tests/e2e/utils.go

@@ -2,15 +2,23 @@ package e2e
 
 import (
 	"context"
+	"flag"
 	"io"
 	"os"
 	"testing"
 	"text/template"
 
+	"github.com/moby/moby/client"
 	"github.com/testcontainers/testcontainers-go"
 	"github.com/testcontainers/testcontainers-go/wait"
 )
 
+var GostBinPath string
+
+func init() {
+	flag.StringVar(&GostBinPath, "gost-bin", "", "Path to a pre-built gost binary (skips compilation)")
+}
+
 type ConfigData struct {
 	ServerAddr string
 }
@@ -70,6 +78,9 @@ func echoContainerRequest(_ context.Context, networkName string) testcontainers.
 			Repo:       "gost-e2e",
 			Tag:        "latest",
 			KeepImage:  true,
+			BuildOptionsModifier: func(opts *client.ImageBuildOptions) {
+				opts.NetworkMode = "host"
+			},
 		},
 		Networks: []string{networkName},
 		NetworkAliases: map[string][]string{
@@ -100,6 +111,9 @@ func udpEchoContainerRequest(_ context.Context, networkName string) testcontaine
 			Repo:       "gost-e2e",
 			Tag:        "latest",
 			KeepImage:  true,
+			BuildOptionsModifier: func(opts *client.ImageBuildOptions) {
+				opts.NetworkMode = "host"
+			},
 		},
 		Networks: []string{networkName},
 		NetworkAliases: map[string][]string{
@@ -134,6 +148,9 @@ func runGostContainer(ctx context.Context, networkName, yamlPath string, aliases
 			Repo:       "gost-e2e",
 			Tag:        "latest",
 			KeepImage:  true,
+			BuildOptionsModifier: func(opts *client.ImageBuildOptions) {
+				opts.NetworkMode = "host"
+			},
 		},
 		ExposedPorts: exposedPorts,
 		// interal check for udp ports will be failed