[infrastructure] Let a suite outside this repo use the e2e harness (#7176)

e2e/harness documents itself as feature-agnostic, but three details
assumed the caller lives in this repo, so the terraform provider's
acceptance suite would otherwise carry a second harness for the same
product.

repoRoot took the first module root above the working directory as the
Docker build context, which from another module is the caller's own
root, with no combined/Dockerfile.multistage in it. It now requires that
ancestor to be this module, and otherwise asks the go tool for the
source: for a dependent, the extracted directory of the version it pins,
so the server matches the client library it was compiled against. That
lookup uses -mod=readonly, since automatic vendor mode otherwise reports
an empty Dir.

Geolocation was disabled unconditionally. Agent-network ingest does not
use it, but location-based posture checks need the database, and a rule
management cannot evaluate fails rather than passing.
StartClient pinned one network alias and set no hostname, so a second
agent could not start and a peer's name was arbitrary. Management
records that hostname, making it the peer's name in the API.
The client entrypoint is copied with an explicit mode: git tracks it
100755, but the module cache extracts 0444, so a dependent's build
produced a container exiting with "permission denied".

Adds CombinedOption, WithGeolocation, WithServerEnv, ClientOption and
WithClientName.

e2e/harness/client.go

@@ -32,12 +32,36 @@ type Client struct {
 	container testcontainers.Container
 }
 
+// clientOptions is what the ClientOption values assemble.
+type clientOptions struct {
+	name string
+}
+
+// ClientOption adjusts how StartClient runs the agent.
+type ClientOption func(*clientOptions)
+
+// WithClientName names the agent, which sets both its network alias and its
+// container hostname. The hostname matters beyond addressing: the agent reports
+// it to management at registration, so it is the name the peer appears under in
+// the API.
+//
+// Required to run more than one agent against the same server — the default name
+// is shared, and two containers cannot hold the same alias on one network.
+func WithClientName(name string) ClientOption {
+	return func(o *clientOptions) { o.name = name }
+}
+
 // StartClient builds the client image and runs it on the combined server's
 // network, joining via the given setup key. The image entrypoint brings the
 // daemon up automatically; callers wait for connectivity with WaitConnected /
 // WaitProxyPeer.
-func StartClient(ctx context.Context, c *Combined, setupKey string) (*Client, error) {
-	root, err := repoRoot()
+func StartClient(ctx context.Context, c *Combined, setupKey string, opts ...ClientOption) (*Client, error) {
+	o := clientOptions{name: clientAlias}
+	for _, opt := range opts {
+		opt(&o)
+	}
+
+	root, err := repoRoot(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -47,9 +71,13 @@ func StartClient(ctx context.Context, c *Combined, setupKey string) (*Client, er
 	}
 
 	req := testcontainers.ContainerRequest{
-		Image:          clientImage,
+		Image: clientImage,
+		// The agent reports the container's hostname to management, so this is
+		// the name the peer is addressable by in the API as well as on the
+		// network. The entrypoint takes no hostname flag of its own.
+		Hostname:       o.name,
 		Networks:       []string{c.network.Name},
-		NetworkAliases: map[string][]string{c.network.Name: {clientAlias}},
+		NetworkAliases: map[string][]string{c.network.Name: {o.name}},
 		Env: map[string]string{
 			"NB_MANAGEMENT_URL": combinedExposedURL,
 			"NB_SETUP_KEY":      setupKey,