Use sha256 hashes for Docker base images to ensure repeatability

Closes gh-15056

ci/images/spring-boot-ci-image/Dockerfile

@@ -1,4 +1,5 @@
-FROM openjdk:8u181-jdk
+# 1.8.0_181-8u181-b13-1~deb9u1-b13
+FROM openjdk@sha256:1b842a215cf3437c9197f0f47e5c1a12f0a27aed27b84536ee3d8ef6b9b7e19e
 
 RUN apt-get update && \
     apt-get install -y git && \

ci/images/spring-boot-jdk10-ci-image/Dockerfile

@@ -1,4 +1,5 @@
-FROM openjdk:10.0.2-13-jdk
+# 10.0.2+13-Debian-2
+FROM openjdk@sha256:6f481d9b45665cbfd39309ef72dc2afe628ff4721c29d3c5b8852287e6cb8668
 
 RUN apt-get update && \
     apt-get install -y git && \

ci/images/spring-boot-jdk9-ci-image/Dockerfile

@@ -1,4 +1,5 @@
-FROM openjdk:9.0.4-12-jdk
+# 9.0.4+12-Debian-4
+FROM openjdk@sha256:0e48e7e79d9e5637ee5d9c4a4fa2615c7bde69390aab06f7105eb4a8ebec8383
 
 RUN apt-get update && \
     apt-get install -y git && \