Use nproc command to automatically scale to number of cores

[olivier-thatch]

This PR changes the nproc option to be a boolean instead of a numeric. When the nproc option is set to true, the following line is added to the Dockerfile:

ENV MAKE="make -j$(nproc)"

which will cause the make command to automatically scale based on the number of CPU cores returned by the nproc command. nproc is part of GNU Core Utilities and should always be available.

This is a fairly common approach, e.g. https://github.com/search?q=path%3ADockerfile+%22ENV+MAKE%3D%5C%22make+-j%24%28nproc%29%5C%22%22&type=code

[luizkowalski]

hey olivier

when I was testing these changes, i got the following error:

> [build 2/8] RUN bundle install &&   rm -rf ~/.bundle/ "/usr/local/bundle"/ruby/*/cache "/usr/local/bundle"/ruby/*/bundler/gems/*/.git:
31.45 /usr/local/bundle/gems/bundler-4.0.18/lib/bundler/worker.rb:56:in
31.45 'Bundler::Worker#process_queue'
31.45 /usr/local/bundle/gems/bundler-4.0.18/lib/bundler/worker.rb:98:in 'block (2
31.45 levels) in Bundler::Worker#create_threads'
31.45
31.45 An error occurred while installing vernier (1.10.1), and Bundler cannot
31.45 continue.
31.45
31.45 In Gemfile:
31.45   vernier
------
Dockerfile:54
--------------------
  53 |     ENV MAKE="make -j$(nproc)"
  54 | >>> RUN bundle install && \
  55 | >>>   rm -rf ~/.bundle/ "${BUNDLE_PATH}"/ruby/*/cache "${BUNDLE_PATH}"/ruby/*/bundler/gems/*/.git
  56 |
--------------------
ERROR: failed to build: failed to solve: process "/bin/sh -c bundle install &&   rm -rf ~/.bundle/ \"${BUNDLE_PATH}\"/ruby/*/cache \"${BUNDLE_PATH}\"/ruby/*/bundler/gems/*/.git" did not complete successfully: exit code: 5
docker stderr: Nothing written

I believe ENV can't run a shell command; it passes whatever you put into it as a string. It probably works for other people because nothing they are installing cares about make.

I believe a better approach is to use ARGS and MAKEFLAGS:

RUN MAKEFLAGS="-j$(nproc)" bundle install && ....