I run build and get error

[nfqhy]

docker build -t zat .
[+] Building 26.4s (9/12)
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 32B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/ubuntu:14.04 1.4s
=> [1/9] FROM docker.io/library/ubuntu:14.04@sha256:63fce984528cec8714c365919882f8fb64c8a3edf23fdfa0b218a2756125456f 0.0s
=> CACHED [2/9] RUN apt-get update -y -qq && apt-get install -y -qq git-core curl zlib1g-dev build-essential libssl-dev libreadline-dev libyaml-dev libsqlite3-dev sqlite3 libxml2-dev libxslt1-dev libcur 0.0s
=> CACHED [3/9] RUN cd && git clone --depth 1 git://github.com/sstephenson/rbenv.git .rbenv && echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc && echo 'eval "$(rbenv init -)"' >> ~/.bashrc & 0.0s
=> CACHED [4/9] RUN /ROOT/.RBENV/BIN/RBENV install 2.1.5 && /root/.rbenv/bin/rbenv global 2.1.5 0.0s
=> CACHED [5/9] RUN echo "gem: --no-ri --no-rdoc" > ~/.gemrc 0.0s
=> ERROR [6/9] RUN /root/.rbenv/shims/gem install zendesk_apps_tools 25.0s

[6/9] RUN /root/.rbenv/shims/gem install zendesk_apps_tools:
#9 12.97 Successfully installed thor-0.19.4
#9 12.97 Successfully installed rubyzip-1.2.4
#9 13.48 Successfully installed rack-1.6.13
#9 13.48 Building native extensions. This could take a while...
#9 20.40 Successfully installed eventmachine-1.2.7
#9 20.40 Successfully installed daemons-1.3.1
#9 20.40 Building native extensions. This could take a while...
#9 21.25 Successfully installed thin-1.7.2
#9 24.92 ERROR: Error installing zendesk_apps_tools:
#9 24.92 ffi requires Ruby version >= 2.3.
#9 24.92 Successfully installed tilt-2.0.10
#9 24.92 Successfully installed rack-protection-1.5.5
#9 24.92 Successfully installed sinatra-1.4.8
#9 24.92 Successfully installed multipart-post-2.1.1
#9 24.92 Successfully installed faraday-0.9.2
#9 24.92 Successfully installed execjs-2.7.0
#9 24.92 Successfully installed ipaddress_2-0.13.0
#9 24.92 Successfully installed mimemagic-0.3.5

---

executor failed running [/bin/sh -c /root/.rbenv/shims/gem install zendesk_apps_tools]: exit code: 1