Docker base image sha on the file creation, modification date. Docker layers are timestamped.

non-deterministic-builds.sh

echo "FROM openjdk:14-alpine\nRUN echo \${JAVA_HOME}" > Dockerfile
docker build --tag my-java:latest . # No cache,                 Successfully built c60c9be06bcb
docker build --tag my-java:latest . # Fully cached,             Successfully built c60c9be06bcb
docker rmi my-java:latest
docker build --tag my-java:latest . # Totally new image,        Successfully built 514662e21b60
docker build --no-cache --tag my-java:latest . # no-cache flag, Successfully built 364da4d3ef39