Dockerfile: Setting multiple environment variables in single line

Question

I was under the impression that environmental variables could be set on a single line as follows so as to minimize intermediary images.

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

However, running a container based off of this snippet and calling # set |grep RU I see that the variables are not being assigned separately, but are combined into a single string.

RUBY_MAJOR='2.4     RUBY_VERSION 2.4.1     RUBY_DOWNLOAD_SHA256 4fc8a9992de3e90191de369270ea4b6c1b171b7941743614cc50822ddc1fe654     RUBYGEMS_VERSION 2.6.12     BUNDLER_VERSION 1.15.3'

However, if I explicitly set each variable as below, I get the expected output and there are no errors when calling the variables.

ENV RUBY_MAJOR 2.4
ENV RUBY_VERSION 2.4.1
ENV RUBY_DOWNLOAD_SHA256 4fc8a9992de3e90191de369270ea4b6c1b171b7941743614cc50822ddc1fe654
ENV RUBYGEMS_VERSION 2.6.12
ENV BUNDLER_VERSION 1.15.3

Question: Is it is possible to combine the setting of environment variables on a single line? If so, how would I do it? And is it a good practice?

Answer 1

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

ENX 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