Pin Blueocean plugin version to 1.6.1

Newer Blueocean plugin versions requires Jenkins >= 2.107.2, but some
plugins are still incompatible with new Jenkins due to JEP-200 issue.

So Blueocean plugin version is pinned instead of Jenkins version bumping.

Change-Id: I4b6b1ef599131f98fcb6f6761e92b673fadcb716
diff --git a/Dockerfile b/Dockerfile
index d368c14..b32109e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -36,6 +36,11 @@
 
 # jenkins version being bundled in this docker image
 ARG JENKINS_VERSION
+# FIXME: Blueocean plugin >= 1.6.2 requires Jenkins >= 2.107, but some plugins are still
+# incompatibble with new Jenkins due to JEP-200 issue. For example performance plugin
+# will be fixed in 3.11 but there is 3.10 at the moment.
+# See: https://wiki.jenkins.io/display/JENKINS/Plugins+affected+by+fix+for+JEP-200
+# Thus Blueocean plugin version is pinned to 1.6.1 below.
 ENV JENKINS_VERSION ${JENKINS_VERSION:-2.104}
 
 # jenkins.war checksum, download will be validated using it
@@ -77,7 +82,7 @@
 RUN JENKINS_UC_DOWNLOAD=http://archives.jenkins-ci.org /usr/local/bin/install-plugins.sh \
         antisamy-markup-formatter \
         artifactory \
-        blueocean \
+        blueocean:1.6.1 \
         build-blocker-plugin \
         build-monitor-plugin \
         build-timeout \