Upgrade to the new LTS baseline: 2.19.1

See also https://jenkins.io/changelog-stable/#v2.19.1
diff --git a/Dockerfile b/Dockerfile
index 90a733d..28c4e31 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -36,10 +36,10 @@
 
 # jenkins version being bundled in this docker image
 ARG JENKINS_VERSION
-ENV JENKINS_VERSION ${JENKINS_VERSION:-2.7.4}
+ENV JENKINS_VERSION ${JENKINS_VERSION:-2.19.1}
 
 # jenkins.war checksum, download will be validated using it
-ARG JENKINS_SHA=07a2e3e4ace728fdbcc823f46068d2f8cc3cb97b
+ARG JENKINS_SHA=dc28b91e553c1cd42cc30bd75d0f651671e6de0b
 
 # Can be used to customize where jenkins.war get downloaded from
 ARG JENKINS_URL=http://repo.jenkins-ci.org/public/org/jenkins-ci/main/jenkins-war/${JENKINS_VERSION}/jenkins-war-${JENKINS_VERSION}.war