Validate file contents when loading files to env variables

[vovimayhem]

I've got this error on one of my apps:

Puma starting in single mode...
* Version 4.3.6 (ruby 2.7.1-p83), codename: Mysterious Traveller
* Min threads: 25, max threads: 25
* Environment: production
! Unable to load application: ArgumentError: bad environment variable value: contains null byte
/usr/local/bundle/gems/on_container-0.0.11/lib/on_container/load_env_secrets.rb:24:in `[]=': bad environment variable value: contains null byte (ArgumentError)
        from /usr/local/bundle/gems/on_container-0.0.11/lib/on_container/load_env_secrets.rb:24:in `block in <top (required)>'
        from /usr/local/bundle/gems/on_container-0.0.11/lib/on_container/load_env_secrets.rb:17:in `each'
        from /usr/local/bundle/gems/on_container-0.0.11/lib/on_container/load_env_secrets.rb:17:in `<top (required)>'
        from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:168:in `require'
        from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:168:in `rescue in require'
        from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:156:in `require'
        from /srv/prex-surescripts/config/boot.rb:5:in `<top (required)>'
        from /srv/prex-surescripts/config/application.rb:3:in `require_relative'
        from /srv/prex-surescripts/config/application.rb:3:in `<top (required)>'
        from /srv/prex-surescripts/config/environment.rb:4:in `require_relative'
        from /srv/prex-surescripts/config/environment.rb:4:in `<top (required)>'
        from config.ru:5:in `require_relative'
        from config.ru:5:in `block in <main>'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:116:in `eval'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:116:in `new_from_string'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:105:in `load_file'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:66:in `parse_file'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/configuration.rb:321:in `load_rackup'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/configuration.rb:246:in `app'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/runner.rb:155:in `load_and_bind'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/single.rb:98:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/launcher.rb:172:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/cli.rb:80:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/bin/puma:10:in `<top (required)>'
        from /usr/local/bundle/bin/puma:23:in `load'
        from /usr/local/bundle/bin/puma:23:in `<main>'
/usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': cannot load such file -- on_container/load_env_secrets (LoadError)
        from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
        from /srv/prex-surescripts/config/boot.rb:5:in `<top (required)>'
        from /srv/prex-surescripts/config/application.rb:3:in `require_relative'
        from /srv/prex-surescripts/config/application.rb:3:in `<top (required)>'
        from /srv/prex-surescripts/config/environment.rb:4:in `require_relative'
        from /srv/prex-surescripts/config/environment.rb:4:in `<top (required)>'
        from config.ru:5:in `require_relative'
        from config.ru:5:in `block in <main>'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:116:in `eval'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:116:in `new_from_string'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:105:in `load_file'
        from /usr/local/bundle/gems/rack-2.2.3/lib/rack/builder.rb:66:in `parse_file'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/configuration.rb:321:in `load_rackup'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/configuration.rb:246:in `app'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/runner.rb:155:in `load_and_bind'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/single.rb:98:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/launcher.rb:172:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/lib/puma/cli.rb:80:in `run'
        from /usr/local/bundle/gems/puma-4.3.6/bin/puma:10:in `<top (required)>'
        from /usr/local/bundle/bin/puma:23:in `load'
        from /usr/local/bundle/bin/puma:23:in `<main>'

Looks like we should be checking the contents of the file before loading it to an env variable at on_container/load_env_secrets