This is the first instruction in the Dockerfile. It sets the base image for every subsequent instruction coming next in the file. The syntax for the FROM instruction is straightforward. It's just:
FROM <image>, or FROM <image>:<tag>, or FROM <image>@<digest>
The FROM instruction takes a tag or digest as a parameter. If you decide to skip them, Docker will assume you want to build your image from the latest tag. Be aware that latest will not always be the latest version of the image you want to build upon. The latest tag is kind of a special one. Also, it may not work as you may expect. Well, to cut a long story short, it doesn't mean anything special unless the image creator (openjdk or fabric8, for example) has a specific build, tag ...
