Default locations are not same as bin and documentation

[uCatu]

See docs:
https://docs.fluentd.org/v/0.12/deployment/plugin-management#add-a-plugin-via-etc-fluent-plugin
Referring to /etc/fluent/plugin

While in code we are using -p /fluentd/plugins

fluentd-docker-image/entrypoint.sh.erb

Line 20 in c848dfe

set -- "$@" -c /fluentd/etc/${FLUENTD_CONF}

Same goes to conf file, when trying to use default binary:

	9: from /usr/bin/fluentd:23:in `<main>'
	8: from /usr/bin/fluentd:23:in `load'
	7: from /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/bin/fluentd:8:in `<top (required)>'
	6: from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:72:in `require'
	5: from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:72:in `require'
	4: from /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/lib/fluent/command/fluentd.rb:344:in `<top (required)>'
	3: from /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/lib/fluent/supervisor.rb:727:in `configure'
	2: from /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/lib/fluent/config.rb:31:in `build'
	1: from /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/lib/fluent/config.rb:31:in `open'
/usr/lib/ruby/gems/2.7.0/gems/fluentd-1.12.1/lib/fluent/config.rb:31:in `initialize': No such file or directory @ rb_sysopen - /etc/fluent/fluent.conf (Errno::ENOENT)```

[kenhys]

https://docs.fluentd.org/v/0.12/deployment/plugin-management#add-a-plugin-via-etc-fluent-plugin
is referred for general purpose, in the other hand, the docker image does not follow it. It seems it is an intentional change (I guess)
beause of docker image is a one of distribution of fluentd.
So, it may be better to mention it in https://github.com/fluent/fluentd-docker-image/blob/master/README.md.