Update dendrite build used to check homerunner (#636)

Author: Mathieu Velten

.github/workflows/ci.yaml

@@ -23,9 +23,12 @@ jobs:
         run: |
           go test ./internal/...
       - name: "Run Homerunner tests" # use a simple static dendrite image to sanity check homerunner works
+        env:
+          DOCKER_BUILDKIT: 1
         run: |
           mkdir -p homeserver
-          wget -O - "https://github.com/matrix-org/dendrite/archive/v0.8.8.tar.gz" | tar -xz --strip-components=1 -C homeserver
+          # Latest official dendrite release is still using Debian Stretch as a base, hence the specific commit
+          wget -O - "https://github.com/matrix-org/dendrite/archive/0489d16f95a3d9f1f5bc532e2060bd2482d7b156.tar.gz" | tar -xz --strip-components=1 -C homeserver
           (cd homeserver && docker build -t complement-dendrite -f build/scripts/Complement.Dockerfile .)
           (cd cmd/homerunner/test && ./test.sh)