Dockerfile: Setting multiple environment variables in single line

There are two formats for specifying environments. If you need single variable then you below format

ENV X Y

This will assign X as Y

ENV X Y Z

This will assign X as Y Z

If you need to assign multiple environment variables then you use the other format

ENV X=Y Z=A

This will assign X as Y and Z as A. So your Dockerfile should be

FROM alpine:3.6
ENV RUBY_MAJOR=2.4 \
    RUBY_VERSION=2.4.1 \
    RUBY_DOWNLOAD_SHA256=4fc8a9992de3e90191de369270ea4b6c1b171b7941743614cc50822ddc1fe654 \
    RUBYGEMS_VERSION=2.6.12 \
    BUNDLER_VERSION=1.15.3

RUN env