Add retry logic to TestFlyDeploy_Dockerfile for SSH failures

The test was failing due to transient WireGuard API errors when
establishing SSH tunnels:
  Error: ssh: can't build tunnel: failed to run mutation addWireGuardPeer:
  server returned a non-200 status code: 500

Changed to use FlyAllowExitFailure instead of Fly, so the EventuallyWithT
retry logic can handle these transient failures. The test now retries SSH
connections for up to 30 seconds (15 attempts @ 2s intervals).

This makes the test more resilient to temporary backend issues while
still validating that the deployed app has the correct environment.

Author: mattp-fly

test/preflight/fly_deploy_test.go

@@ -175,7 +175,12 @@ ENV PREFLIGHT_TEST=true`)
 	f.Fly("launch --org %s --name %s --region %s --internal-port 80 --ha=false --now", f.OrgSlug(), appName, f.PrimaryRegion())
 
 	require.EventuallyWithT(t, func(c *assert.CollectT) {
-		sshResult := f.Fly("ssh console -C 'printenv PREFLIGHT_TEST'")
+		// Use FlyAllowExitFailure to handle transient WireGuard API failures (HTTP 500)
+		sshResult := f.FlyAllowExitFailure("ssh console -C 'printenv PREFLIGHT_TEST'")
+		if sshResult.ExitCode() != 0 {
+			assert.Fail(c, "ssh command failed, will retry", "exit code: %d, stderr: %s", sshResult.ExitCode(), sshResult.StdErrString())
+			return
+		}
 		assert.Equal(c, "true", strings.TrimSpace(sshResult.StdOutString()), "expected PREFLIGHT_TEST env var to be set in machine")
 	}, 30*time.Second, 2*time.Second)
 }