Best practice for dockerfile maintain?

Question

I have a Dockerfile something like follows:

FROM openjdk:8u151

# others here

I have 2 questions about the base image:

1

Answer 1

How to get the tags?

See "How to list all tags for a Docker image on a remote registry?".
The API is enough

For instance, visit:

https://registry.hub.docker.com/v2/repositories/library/java/tags/?page_size=100&page=2

Will the base image safe?

As long as you save your own built image in a registry (eithe rpublic one, or a self-hosted one), yes: you will be able to at least build new images based on the one you have done.
Or, even if the base image disappears, you still have its layers in your own image, and can re-tag it (provided the build cache is available).
See for instance "Is there a way to tag a previous layer in a docker image or revert a commit?".
See caveats in "can I run an intermediate layer of docker image?".