Pin Docker image ...2.3-jdk-8-groovy-2.4.3-onbuild

saturnism/spring-boot:1.2.3-jdk-8-groovy-2.4.3-onbuild
->
saturnism/spring-boot:1.2.3-jdk-8-groovy-2.4.3-onbuild@sha256:959654a406a98d71197bea48dd9e177d661d588af2f5439ff8b28e7f11065045

[atomist:generated]
[atomist-skill:atomist/docker-base-image-policy]

Author: atomist-bot

examples/guestbook-service/Dockerfile

@@ -14,4 +14,4 @@
 # limitations under the License.
 ###############################################################################
 
-FROM saturnism/spring-boot:1.2.3-jdk-8-groovy-2.4.3-onbuild
+FROM saturnism/spring-boot:1.2.3-jdk-8-groovy-2.4.3-onbuild@sha256:959654a406a98d71197bea48dd9e177d661d588af2f5439ff8b28e7f11065045