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.

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/go-gost/core v0.4.1
 	github.com/go-gost/x v0.10.11-0.20260605152603-e45d9a8cc81a
 	github.com/judwhite/go-svc v1.2.1
+	github.com/moby/moby/client v0.4.0
 	github.com/stretchr/testify v1.11.1
 	github.com/testcontainers/testcontainers-go v0.42.0
 )
@@ -83,7 +84,6 @@ require (
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/go-archive v0.2.0 // indirect
 	github.com/moby/moby/api v1.54.1 // indirect
-	github.com/moby/moby/client v0.4.0 // indirect
 	github.com/moby/patternmatcher v0.6.1 // indirect
 	github.com/moby/sys/sequential v0.6.0 // indirect
 	github.com/moby/sys/user v0.4.0 // indirect