Give a container the names, rather than a resolver to ask

The commit before this said "told where to resolve names" and passed --dns,
which is not what it ended up doing. This is that correction: a container is
given the names themselves, written into its own hosts file by the runtime.

The reason for the change is the decision the mesh already made about names — a
file rather than a resolver, because it works on every runtime, needs no
package and has no failure mode of its own. Passing a resolver address would
have required a resolver to exist, which at that point none did.

A resolver is coming, for the case a file genuinely cannot express: a service
named under a machine, postgres.novox.internal, where the wildcard cannot be
enumerated in advance. When it arrives it will need this field back under its
own name. It is not being kept in the meantime — a field nothing fills is a
field nobody can trust, and the vocabulary is asserted by a count for exactly
that reason.

internal/apply/apply.go

@@ -812,12 +812,12 @@ func applyContainer(ctx context.Context, r *declaration.Container, run Runner) (
 	for _, v := range r.Volumes {
 		args = append(args, "--volume", v)
 	}
-	for _, n := range r.Nameservers {
-		// Per container rather than by changing the machine's resolver configuration. That file
-		// belongs to something else on most machines, and a host that edited it would be fighting
-		// whatever owns it on every boot — the fault this host exists to avoid, in the one place
-		// it would be hardest to see.
-		args = append(args, "--dns", n)
+	for _, h := range r.Hosts {
+		// Written into the container's own hosts file by the runtime. Per container rather than
+		// by editing the machine's resolver configuration: that file belongs to something else on
+		// most machines, and a host that edited it would be fighting whatever owns it on every
+		// boot — the fault this host exists to avoid, in the place it would be hardest to see.
+		args = append(args, "--add-host", h)
 	}
 	args = append(args, r.Image)
 	args = append(args, r.Args...)

internal/apply/apply_test.go

@@ -1231,7 +1231,7 @@ func TestAFailedActionStopsWhatFollows(t *testing.T) {
 // `resolv.conf` — so every internal name the mesh wrote for the machine is invisible to what the
 // machine is running. That was hit for real: a database client on one node could not resolve
 // another node, on a mesh where both names were correct and present on both machines.
-func TestAContainerIsToldWhichResolverToUse(t *testing.T) {
+func TestAContainerIsGivenTheMeshsNames(t *testing.T) {
 	var ran []string
 	run := func(_ context.Context, name string, args ...string) (string, error) {
 		if name != "docker" {
@@ -1250,25 +1250,23 @@ func TestAContainerIsToldWhichResolverToUse(t *testing.T) {
 	}
 	d := parseTrusted(t, `{"declaration":1,"resources":[
 	  {"id":"app","type":"container","name":"app","image":"`+pinned+`",
-	   "nameservers":["10.42.0.1"]}
+	   "hosts":["anchor.internal:10.42.0.1"]}
 	]}`)
 	_, _, _ = Apply(context.Background(), archHost(t), d, store.State{}, store.OriginCarried, run, nil, nil)
 
 	var told bool
 	for i, a := range ran {
-		if a == "--dns" && i+1 < len(ran) && ran[i+1] == "10.42.0.1" {
+		if a == "--add-host" && i+1 < len(ran) && ran[i+1] == "anchor.internal:10.42.0.1" {
 			told = true
 		}
 	}
 	if !told {
-		t.Fatalf("the container was not told where to resolve names: %v", ran)
+		t.Fatalf("the container cannot reach another machine by name: %v", ran)
 	}
 }
 
-// And a container that was told nothing is run exactly as before: most containers resolve
-// whatever the machine resolves, and passing an empty flag would be a change of behaviour
-// dressed as a default.
-func TestAContainerToldNothingIsRunAsBefore(t *testing.T) {
+// And a container given no names is run exactly as before.
+func TestAContainerGivenNoNamesIsRunAsBefore(t *testing.T) {
 	var ran []string
 	run := func(_ context.Context, name string, args ...string) (string, error) {
 		if name != "docker" {
@@ -1291,8 +1289,8 @@ func TestAContainerToldNothingIsRunAsBefore(t *testing.T) {
 	_, _, _ = Apply(context.Background(), archHost(t), d, store.State{}, store.OriginCarried, run, nil, nil)
 
 	for _, a := range ran {
-		if a == "--dns" {
-			t.Fatalf("a container that was told nothing was given a resolver anyway: %v", ran)
+		if a == "--add-host" {
+			t.Fatalf("a container given no names was given some anyway: %v", ran)
 		}
 	}
 }

internal/declaration/declaration.go

@@ -328,18 +328,22 @@ type Container struct {
 	Ports   []string          `json:"ports,omitempty"`
 	Volumes []string          `json:"volumes,omitempty"`
 	Args    []string          `json:"args,omitempty"`
-	// Nameservers this container resolves through.
+	// Names this container can reach, as `name:address`.
 	//
 	// **Because a container does not inherit the machine's names.** It gets its own `/etc/hosts`
-	// holding its own hostname, and a runtime rewrites `resolv.conf` — so every internal name the
-	// mesh wrote for this machine is invisible to the thing the machine is running. That was hit
-	// for real: a database client on one node could not resolve another node, on a mesh where
-	// both names were correct and present.
+	// holding only its own hostname, so every internal name the mesh wrote for this machine is
+	// invisible to the thing the machine is running. That was hit for real: a database client on
+	// one node could not resolve another node, on a mesh where both names were correct and
+	// present on both machines.
 	//
-	// Set by the mesh rather than by a module: which resolver a machine has is a fact about the
-	// machine, and a module that named one would be a module that only runs where somebody put
-	// that resolver.
-	Nameservers []string `json:"nameservers,omitempty"`
+	// **A file rather than a resolver, which is the decision the mesh already made about names**
+	// and this extends rather than overturns: it works on every runtime, needs no package, and
+	// has no failure mode of its own. A resolver becomes necessary when names are wanted that are
+	// not one-per-node — service names, wildcards — and that is still not true.
+	//
+	// Set by the mesh, not by a module: which machines exist is a fact about the mesh, and a
+	// module that listed them would be a module that goes stale when one joins.
+	Hosts []string `json:"hosts,omitempty"`
 
 	// Network is the container's network, passed to the runtime unchanged.
 	//