feat: add Prometheus metrics endpoint and Docker image (PLAN.md 4.2)

Add internal/metrics package with dedicated Prometheus registry exposing
SSH connection, auth attempt, session, and build info metrics. Wire into
SSH server (4 instrumentation points) and web server (/metrics endpoint).
Add dockerImage output to flake.nix via dockerTools.buildLayeredImage.
Bump version to 0.7.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -24,13 +24,30 @@
             pname = "oubliette";
             inherit version;
             src = ./.;
-            vendorHash = "sha256-oH92jRD+2niIf7xAX1HeZvhux8lVqj43Qxdef5GjX4Q=";
+            vendorHash = "sha256-smMg/J1igSoSBkzdm9HJOp5OYY8MEccodCD/zVK31IQ=";
             subPackages = [ "cmd/oubliette" ];
             meta = {
               description = "SSH honeypot";
               mainProgram = "oubliette";
             };
           };
+
+          dockerImage = pkgs.dockerTools.buildLayeredImage {
+            name = "oubliette";
+            tag = version;
+            contents = [ self.packages.${system}.default ];
+            config = {
+              Entrypoint = [ "/bin/oubliette" ];
+              Cmd = [ "-config" "/data/oubliette.toml" ];
+              ExposedPorts = {
+                "2222/tcp" = {};
+                "8080/tcp" = {};
+              };
+              Volumes = {
+                "/data" = {};
+              };
+            };
+          };
         });
 
       devShells = forAllSystems (system: