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>

.github/workflows/test.yml

@@ -24,6 +24,9 @@ jobs:
         with:
           go-version: "1.26"
           cache: true
+          cache-dependency-path: |
+            go.sum
+            test/e2e/go.sum
 
       - name: Format
         run: |
@@ -39,3 +42,13 @@ jobs:
 
       - name: Test
         run: go test ./...
+
+      # The e2e module runs only in release.yml, after a release is published;
+      # a type error there would first show up then. Compile it here without
+      # running it: `go test -run` would still run TestMain, which brings up
+      # the whole stand.
+      - name: Compile the e2e
+        working-directory: test/e2e
+        run: |
+          go vet ./...
+          go test -c -o /dev/null .