https://docs.docker.com/engine/reference/builder/#copy---link has a text that says:
Added in
docker/dockerfile:1.4
This refers to https://docs.docker.com/build/dockerfile/frontend/. An example of its usage:
# syntax=docker/dockerfile:4
FROM alpine
COPY --link /foo /bar
But how do I know which current version of frontend is used if I do not specify the syntax specifically?