Bind 0.0.0.0 to IPv4 only (closes #528)

Go listens on any unspecified address with one socket that answers
IPv4 and IPv6, so BIND_ADDRESS=0.0.0.0 also bound every IPv6 address,
which the README says it does not. The listener now uses tcp4 for an
IPv4 address, written plain or IPv4-mapped; :: still binds both. The
server's start hook suppresses contextcheck, as the other start hooks
do.

The image's health check asked for localhost, which wget reached over
::1 only. It now asks for the container's own address: its IPv4
address, or its IPv6 address on an IPv6-only network. A loopback bind
answers neither. The README says such a network needs ::, drops the
bindv6only condition from :: (Go sets that socket option itself) and
names an ss check.

Model: opus-5-5

internal/server/http.go

@@ -1,9 +1,11 @@
 package server
 
 import (
+	"context"
 	"errors"
 	"net"
 	"net/http"
+	"net/netip"
 	"strconv"
 	"time"
 )
@@ -39,6 +41,23 @@ func (s *Server) listenAddr() string {
 	)
 }
 
+// listenNetwork returns the network the HTTP listener binds on, "tcp4"
+// for an IPv4 BindAddress and "tcp" for any other.
+//
+// On "tcp", Go binds 0.0.0.0 with a socket that answers IPv6 as well,
+// exactly as it binds ::. "tcp4" keeps 0.0.0.0 to IPv4, which is what
+// the README says it means. :: stays on "tcp" and answers both. An
+// IPv4-mapped address such as ::ffff:0.0.0.0 is the IPv4 address it
+// carries to Go's listener, so it gets "tcp4" too.
+func (s *Server) listenNetwork() string {
+	addr, err := netip.ParseAddr(s.params.Config.BindAddress)
+	if err == nil && addr.Unmap().Is4() {
+		return "tcp4"
+	}
+
+	return "tcp"
+}
+
 // newHTTPServer builds the HTTP server for this Server's
 // configuration.
 //
@@ -67,7 +86,17 @@ func (s *Server) serveUntilShutdown() {
 
 	s.log.Info("http begin listen", "listenaddr", s.httpServer.Addr)
 
-	err := s.httpServer.ListenAndServe()
+	// ListenAndServe always listens on "tcp", which would undo
+	// listenNetwork.
+	var listenCfg net.ListenConfig
+
+	listener, err := listenCfg.Listen(
+		context.Background(), s.listenNetwork(), s.httpServer.Addr,
+	)
+	if err == nil {
+		err = s.httpServer.Serve(listener)
+	}
+
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {
 		s.log.Error("listen error", "error", err)
 		s.shutdownWithFailure()