How does this Dockerfile actually run logstash without an entrypoint or cmd?

Question

Just doing a container start on this official logstash docker container does make logstash properly run, given the right config.

It does not have an entrypoint or cmd, or anything of the sort though. I am also not issuing one on the start command. So, how is logstash actually getting executed in this case?

I need to know because I need to edit the command for other reasons. We're working on running it in kubernetes but are just testing with local docker for now.

https://github.com/elastic/logstash/blob/7.15/Dockerfile

Copied for easy reference:

FROM ubuntu:bionic

RUN apt-get update && \
    apt-get install -y zlib1g-dev build-essential vim rake git curl libssl-dev libreadline-dev libyaml-dev  \
      libxml2-dev libxslt-dev openjdk-11-jdk-headless curl iputils-ping netcat && \
    apt-get clean

WORKDIR /root

RUN adduser --disabled-password --gecos "" --home /home/logstash logstash && \
    mkdir -p /usr/local/share/ruby-build && \
    mkdir -p /opt/logstash && \
    mkdir -p /opt/logstash/data && \
    mkdir -p /mnt/host && \
    chown logstash:logstash /opt/logstash

USER logstash
WORKDIR /home/logstash

# used by the purge policy
LABEL retention="keep"

# Setup gradle wrapper. When running any `gradle` command, a `settings.gradle` is expected (and will soon be required).
# This section adds the gradle wrapper, `settings.gradle` and sets the permissions (setting the user to root for `chown`
# and working directory to allow this and then reverts back to the previous working directory and user.
COPY --chown=logstash:logstash gradlew /opt/logstash/gradlew
COPY --chown=logstash:logstash gradle/wrapper /opt/logstash/gradle/wrapper
COPY --chown=logstash:logstash settings.gradle /opt/logstash/settings.gradle
WORKDIR /opt/logstash
RUN for iter in `seq 1 10`; do ./gradlew wrapper --warning-mode all && exit_code=0 && break || exit_code=$? && echo "gradlew error: retry $iter in 10s" && sleep 10; done; exit $exit_code
WORKDIR /home/logstash

ADD versions.yml /opt/logstash/versions.yml
ADD LICENSE.txt /opt/logstash/LICENSE.txt
ADD NOTICE.TXT /opt/logstash/NOTICE.TXT
ADD licenses /opt/logstash/licenses
ADD CONTRIBUTORS /opt/logstash/CONTRIBUTORS
ADD Gemfile.template Gemfile.jruby-2.5.lock.* /opt/logstash/
ADD Rakefile /opt/logstash/Rakefile
ADD build.gradle /opt/logstash/build.gradle
ADD rubyUtils.gradle /opt/logstash/rubyUtils.gradle
ADD rakelib /opt/logstash/rakelib
ADD config /opt/logstash/config
ADD spec /opt/logstash/spec
ADD qa /opt/logstash/qa
ADD lib /opt/logstash/lib
ADD pkg /opt/logstash/pkg
ADD tools /opt/logstash/tools
ADD logstash-core /opt/logstash/logstash-core
ADD logstash-core-plugin-api /opt/logstash/logstash-core-plugin-api
ADD bin /opt/logstash/bin
ADD modules /opt/logstash/modules
ADD x-pack /opt/logstash/x-pack
ADD ci /opt/logstash/ci

USER root
RUN rm -rf build && \
    mkdir -p build && \
    chown -R logstash:logstash /opt/logstash
USER logstash
WORKDIR /opt/logstash

LABEL retention="prune"

Answer 1

If you look at the final layer on the image here , it looks like there is an ENTRYPOINT ["/usr/local/bin/docker-entrypoint"] . The Dockerfile you've linked might not be the one used to build the image.