ci: compile the e2e on every push; test/ out of the build context (audit M20, L33)

- test.yml: after go test, `go vet ./...` and `go test -c -o /dev/null .`
  in test/e2e - compiled and linked, not run. Not `go test -run xxx` as
  the plan said: TestMain would still run and bring up the stand. The Go
  cache now keys on test/e2e/go.sum too.
- .dockerignore: test (e2e leftovers in test/e2e/.stage - DKIM keys,
  sasldb2 - no longer reach the builder; the sink image is built from
  test/e2e/sink as its own context, so it is unaffected).
- development.md (test.yml line), CHANGELOG [Unreleased], audit plan
  lines ticked, roadmap 3/61.

Evidence on the test server for this tree: make e2e (no
SELFPOST_E2E_VERSION, so the "e2e" default path) exit 0, including
image_stamped_with_version and the logrotate-mode build; in test/e2e
go vet ./... and go test -c -o /dev/null . clean; docker build of
build/Dockerfile with a file under test/e2e/.stage succeeds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development.md

@@ -428,7 +428,9 @@ Workflows in [.github/workflows/](../.github/workflows/). What each job runs —
 
 `design-first` (needs the full history) → `gofmt -l` → `go vet ./...` →
 `go test ./...` (main module, no e2e; includes the panel guard tests, see
-[§ Panel pages](#panel-pages-the-component-kit-and-its-guards)).
+[§ Panel pages](#panel-pages-the-component-kit-and-its-guards)) → the e2e
+module compiled, not run (`go vet ./...` and `go test -c -o /dev/null .` in
+`test/e2e`).
 
 ### `release.yml` — published GitHub Release, or `workflow_dispatch` with SemVer