Add docker-compose to the slave-bazel-docker image

'task' plugin tests rely on having docker-compose available to run tests
against a running Gerrit server.

Change-Id: Ibd55bdbb0a2d2d3dd334d1919089669906a00a7f
diff --git a/jenkins-docker/slave-bazel-docker/Dockerfile b/jenkins-docker/slave-bazel-docker/Dockerfile
index 997fe2b..57c6e0c 100644
--- a/jenkins-docker/slave-bazel-docker/Dockerfile
+++ b/jenkins-docker/slave-bazel-docker/Dockerfile
@@ -7,3 +7,8 @@
       stable" && \
     apt-get update && \
     apt-get install -y docker-ce git-
+
+RUN curl -L \
+    "https://github.com/docker/compose/releases/download/1.29.2/docker-compose-$(uname -s)-$(uname -m)" \
+    -o /usr/local/bin/docker-compose && \
+    chmod +x /usr/local/bin/docker-compose