Adds 12 new HTTP handler test scenarios: HTTP connector (no-auth, auth, TLS
upstream), CONNECT tunnel (no-sniffing, bypass-403), probeResist (host, web,
file, knock), idleTimeout, and UDP relay over HTTP. Introduces
RunGostContainerWithFiles helper for mounting extra files into test containers.
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.