can't install "digest" dependency as an unpriviledged user

This is similar to #24, which failed due to io-wait. In this case https://github.com/ruby/digest/issues/14 prevents installation unless the user has access to update the header install location.

some initial setup to create an unpriviledged user:

$ docker run --rm -it ruby:2.7.4 bash
root@d216e60e0a0c:/# useradd --create-home --shell /bin/bash --user-group user
root@d216e60e0a0c:/# su - user
user@d216e60e0a0c:~$ export GEM_HOME="$HOME/gems/$RUBY_MAJOR"
user@d216e60e0a0c:~$ export GEM_PATH="${GEM_HOME}:${GEM_PATH}"

Now, to prove that the user can install other gems and can compile extensions:

user@d216e60e0a0c:~$ gem install json
Fetching json-2.6.1.gem
Building native extensions. This could take a while...
Successfully installed json-2.6.1
1 gem installed

However, net-imap cannot be installed.

user@d216e60e0a0c:~$ gem install net-imap
Fetching net-imap-0.2.2.gem
Fetching io-wait-0.2.1.gem
Fetching net-protocol-0.1.2.gem
Fetching digest-3.0.0.gem
Building native extensions. This could take a while...
Successfully installed io-wait-0.2.1
Successfully installed net-protocol-0.1.2
Building native extensions. This could take a while...
ERROR:  Error installing net-imap:
        ERROR: Failed to build gem native extension.

    current directory: /home/user/gems/gems/digest-3.0.0/ext/digest
/usr/local/bin/ruby -I /usr/local/lib/ruby/2.7.0 -r ./siteconf20211119-19-e2jvw8.rb extconf.rb
creating Makefile

current directory: /home/user/gems/gems/digest-3.0.0/ext/digest
make "DESTDIR=" clean

current directory: /home/user/gems/gems/digest-3.0.0/ext/digest
make "DESTDIR="
compiling digest.c
linking shared-object digest.so

current directory: /home/user/gems/gems/digest-3.0.0/ext/digest
make "DESTDIR=" install
/usr/bin/install -c -m 0755 digest.so ./.gem.20211119-19-ted4vt
installing digest libraries
/usr/bin/install: cannot remove '/usr/local/include/ruby-2.7.0/ruby/digest.h': Permission denied
make: *** [Makefile:204: /usr/local/include/ruby-2.7.0/ruby/digest.h] Error 1

make install failed, exit code 2

Gem files will remain installed in /home/user/gems/gems/digest-3.0.0 for inspection.
Results logged to /home/user/gems/extensions/x86_64-linux/2.7.0/digest-3.0.0/gem_make.out

And the latest pre-release doesn't fix the issue either:

user@d216e60e0a0c:~$ gem install digest:3.1.0.pre3
Fetching digest-3.1.0.pre3.gem
Building native extensions. This could take a while...
ERROR:  Error installing digest:
        ERROR: Failed to build gem native extension.

    current directory: /home/user/gems/gems/digest-3.1.0.pre3/ext/digest
/usr/local/bin/ruby -I /usr/local/lib/ruby/2.7.0 -r ./siteconf20211119-179-agra2q.rb extconf.rb
creating Makefile

current directory: /home/user/gems/gems/digest-3.1.0.pre3/ext/digest
make "DESTDIR=" clean

current directory: /home/user/gems/gems/digest-3.1.0.pre3/ext/digest
make "DESTDIR="
compiling digest.c
linking shared-object digest.so

current directory: /home/user/gems/gems/digest-3.1.0.pre3/ext/digest
make "DESTDIR=" install
/usr/bin/install -c -m 0755 digest.so ./.gem.20211119-179-904e0d
installing header files
install -c -m 644 ./digest.h /usr/local/include/ruby-2.7.0/ruby
/usr/local/lib/ruby/2.7.0/fileutils.rb:1415:in `initialize': Permission denied @ rb_sysopen - /usr/local/include/ruby-2.7.0/ruby/digest.h (Errno::EACCES)
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1415:in `open'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1415:in `block in copy_file'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1414:in `open'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1414:in `copy_file'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:511:in `copy_file'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:880:in `block in install'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1589:in `block in fu_each_src_dest'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1603:in `fu_each_src_dest0'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:1587:in `fu_each_src_dest'
        from /usr/local/lib/ruby/2.7.0/fileutils.rb:876:in `install'
        from install_headers.rb:9:in `block in <main>'
        from install_headers.rb:8:in `each'
        from install_headers.rb:8:in `<main>'
make: *** [Makefile:283: do-install-headers] Error 1

make install failed, exit code 2

Gem files will remain installed in /home/user/gems/gems/digest-3.1.0.pre3 for inspection.
Results logged to /home/user/gems/extensions/x86_64-linux/2.7.0/digest-3.1.0.pre3/gem_make.out

Reproduce the failure in the ruby:2.7.4 Docker image using the unprivileged-user setup and the net-imap installation command. Start with the digest ext/digest installation output and install_headers.rb, and review the linked ruby/digest issue #14. Done means net-imap and its digest dependency install successfully without elevated permissions.