Fix build recipe execution and flatten reliability

- Set HOME in bctx.EnvVars when USER switches so ~ expands correctly in
  subsequent RUN/WORKDIR steps instead of resolving to /root
- Run /bin/sync inside the guest before FlattenRootfs destroys the VM,
  preventing pip-installed files from being captured as 0-byte due to
  unflushed page cache
- Wrap healthcheck command with su <user> so it runs with the template's
  default user context (correct HOME, correct UID)
- Export Shellescape from the recipe package for use in build service
- Add code-runner-beta recipe (Jupyter server with ipykernel --sys-prefix)
  and replace old python-interpreter-v0-beta

internal/recipe/context.go

@@ -115,8 +115,11 @@ func expandEnv(s string, vars map[string]string) string {
 	})
 }
 
-// shellescape wraps s in single quotes, escaping any embedded single quotes.
+// Shellescape wraps s in single quotes, escaping any embedded single quotes.
 // This is POSIX-safe for paths, env values, and shell commands.
-func shellescape(s string) string {
+func Shellescape(s string) string {
 	return "'" + strings.ReplaceAll(s, "'", `'\''`) + "'"
 }
+
+// shellescape is the package-internal alias for Shellescape.
+func shellescape(s string) string { return Shellescape(s) }

internal/recipe/executor.go

@@ -193,6 +193,15 @@ func execUser(
 	entry := execRawShell(ctx, st.Raw, sandboxID, phase, step, 30*time.Second, execFn, script)
 	if entry.Ok {
 		bctx.User = username
+		// Update HOME so ~ expands correctly in subsequent RUN/WORKDIR steps.
+		if bctx.EnvVars == nil {
+			bctx.EnvVars = make(map[string]string)
+		}
+		if username == "root" {
+			bctx.EnvVars["HOME"] = "/root"
+		} else {
+			bctx.EnvVars["HOME"] = "/home/" + username
+		}
 	}
 	return entry, entry.Ok
 }

internal/sandbox/manager.go

@@ -901,6 +901,18 @@ func (m *Manager) FlattenRootfs(ctx context.Context, sandboxID string, teamID, t
 		return 0, fmt.Errorf("sandbox %s not found", sandboxID)
 	}
 
+	// Flush guest page cache to disk before stopping the VM. Without this,
+	// files written by the build (e.g. pip-installed packages) may exist in the
+	// guest's page cache but not yet on the dm block device — flatten would then
+	// capture 0-byte files.
+	func() {
+		syncCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+		defer cancel()
+		if _, err := sb.client.Exec(syncCtx, "/bin/sync"); err != nil {
+			slog.Warn("flatten: guest sync failed (non-fatal)", "id", sb.ID, "error", err)
+		}
+	}()
+
 	// Stop the VM but keep the dm device alive for flattening.
 	m.stopSampler(sb)
 	if err := m.vm.Destroy(ctx, sb.ID); err != nil {

internal/service/build.go

@@ -444,7 +444,7 @@ func (s *BuildService) executeBuild(ctx context.Context, buildIDStr string) {
 			return
 		}
 		log.Info("running healthcheck", "cmd", hc.Cmd, "interval", hc.Interval, "timeout", hc.Timeout, "start_period", hc.StartPeriod, "retries", hc.Retries)
-		if err := s.waitForHealthcheck(buildCtx, agent, sandboxIDStr, hc); err != nil {
+		if err := s.waitForHealthcheck(buildCtx, agent, sandboxIDStr, hc, templateDefaultUser); err != nil {
 			s.destroySandbox(buildCtx, agent, sandboxIDStr)
 			if buildCtx.Err() != nil {
 				return
@@ -544,7 +544,14 @@ func (s *BuildService) executeBuild(ctx context.Context, buildIDStr string) {
 // During the start period, failures are not counted toward the retry budget.
 // Returns nil on the first successful check, or an error if retries are
 // exhausted, the deadline passes, or the context is cancelled.
-func (s *BuildService) waitForHealthcheck(ctx context.Context, agent buildAgentClient, sandboxIDStr string, hc recipe.HealthcheckConfig) error {
+func (s *BuildService) waitForHealthcheck(ctx context.Context, agent buildAgentClient, sandboxIDStr string, hc recipe.HealthcheckConfig, user string) error {
+	// Wrap the healthcheck command with su when a non-root user is set, so that
+	// ~ expands to the correct home directory and the process runs with the
+	// right UID (matching the template's default user).
+	cmd := hc.Cmd
+	if user != "" && user != "root" {
+		cmd = "su " + recipe.Shellescape(user) + " -s /bin/sh -c " + recipe.Shellescape(hc.Cmd)
+	}
 	ticker := time.NewTicker(hc.Interval)
 	defer ticker.Stop()
 
@@ -571,7 +578,7 @@ func (s *BuildService) waitForHealthcheck(ctx context.Context, agent buildAgentC
 			resp, err := agent.Exec(execCtx, connect.NewRequest(&pb.ExecRequest{
 				SandboxId:  sandboxIDStr,
 				Cmd:        "/bin/sh",
-				Args:       []string{"-c", hc.Cmd},
+				Args:       []string{"-c", cmd},
 				TimeoutSec: int32(hc.Timeout.Seconds()),
 			}))
 			cancel()