does it work ?

[danny1144]

Step 2/2 : RUN ["gem", "install", "fluent-plugin-elasticsearch", "--no-rdoc", "--no-ri", "--version", "1.9.5"]
---> Running in b9228068d59a
ERROR: Could not find a valid gem 'fluent-plugin-elasticsearch' (= 1.9.5), here is why:
Unable to download data from https://rubygems.org/ - no such name (https://rubygems.org/specs.4.8.gz)
ERROR: Service 'fluentd' failed to build: The command 'gem install fluent-plugin-elasticsearch --no-rdoc --no-ri --version 1.9.5' returned a non-zero code: 2

[SiM22]

Look at the fluent-elastic plugin GitHub repo and append the latest version to it in the Dockerfile