Allow arbitrary "--user" values (skip "gosu" unless we're running as root)

[tianon]

Fixes #11

See also docker-library/elasticsearch#77

[tianon]

To run as another user, one currently has to use --entrypoint since our use of gosu is unconditional, but this change allows for --user to be used arbitrarily.

[tianon]

Hmm, turns out /var/lib/logstash makes this slightly more complicated.

[tianon]

Sending Logstash's logs to /var/log/logstash which is now configured via log4j2.properties 22:52:37.077 [LogStash::Runner] FATAL logstash.runner - An unexpected error occurred! {:error=>#<ArgumentError: Path "/var/lib/logstash" is not a directory or not writable.>, :backtrace=>["/usr/share/logstash/logstash-core/lib/logstash/settings.rb:396:in initialize'", "org/jruby/RubyProc.java:281:in call'", "/usr/share/logstash/logstash-core/lib/logstash/settings.rb:191:in validate'", "/usr/share/logstash/logstash-core/lib/logstash/settings.rb:182:in validate_value'", "/usr/share/logstash/logstash-core/lib/logstash/settings.rb:100:in validate_all'", "org/jruby/RubyHash.java:1342:in each'", "/usr/share/logstash/logstash-core/lib/logstash/settings.rb:99:in validate_all'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:224:in execute'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67:in run'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:175:in run'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132:in run'", "/usr/share/logstash/lib/bootstrap/environment.rb:68:in (root)'"]}

[tianon]

It does work if I specify --tmpfs /var/lib/logstash as well, so I'm going to chalk that one up to a documentation issue (if you want to use arbitrary --user values, you need to also handle permissions on /var/lib/logstash). Sound fair?

[yosifkit]

Sounds fair to me; just like the MongoDB image.