Bazel: Optimize cache warm-up

This change is optimization of building of slave-bazel docker image,
after reverting of building the whole world in CI build recently:
I87a8fc784a1.

Change-Id: Ib7353dc46a474001fa2a28d7bf5dc21dd7cacd77
diff --git a/jenkins-docker/slave-bazel/Dockerfile b/jenkins-docker/slave-bazel/Dockerfile
index 2cc1de0..69eafbf 100644
--- a/jenkins-docker/slave-bazel/Dockerfile
+++ b/jenkins-docker/slave-bazel/Dockerfile
@@ -7,10 +7,10 @@
     cd /tmp && git clone -b stable-2.16 --recursive https://gerrit.googlesource.com/gerrit && \
     cd /tmp/gerrit && \
     ./tools/setup_gjf.sh 1.7 && \
-    ( bazelisk build //... || true ) && \
-    git checkout -f stable-3.0 && git submodule update --init && ( bazelisk build //... || true ) && \
-    git checkout -f stable-3.1 && git submodule update --init && ( bazelisk build //... || true ) && \
-    git checkout -f master && git submodule update --init && ( bazelisk build //... || true ) && \
+    ( bazelisk build plugins:core release api || true ) && \
+    git checkout -f stable-3.0 && git submodule update --init && ( bazelisk build plugins:core release api || true ) && \
+    git checkout -f stable-3.1 && git submodule update --init && ( bazelisk build plugins:core release api || true ) && \
+    git checkout -f master && git submodule update --init && ( bazelisk build plugins:core release api || true ) && \
     cd /tmp/gerrit && mv tools/format ~ && \
     rm -Rf /tmp/gerrit'