gha/e2e: Also test against moby master

Add a "master" engine-version to the e2e matrix to catch regressions
against the upcoming engine release before it reaches an rc.

There is no "master" dind image, so just copy the engine binary from the
moby/moby-bin:master image and put it into the latest dind image.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

scripts/test/e2e/run

@@ -42,6 +42,11 @@ setup() {
         sh e2e/testdata/registry/certs/gen-certs.sh
     fi
 
+    # ENGINE_VERSION=master tests against moby/moby-bin:master.
+    if [ "${ENGINE_VERSION:-}" = "master" ]; then
+        export ENGINE_SOURCE=moby-bin
+    fi
+
     COMPOSE_PROJECT_NAME=$project COMPOSE_FILE=$file docker compose up --build -d >&2
 
     # Ensure supporting services exist before running tests. If one fails to start,