How to install ElasticSeach plugins using docker compose

Here is a blog post by Elastic pertaining to exactly that! You need to use a Dockerfile which executes commands to extend an image. Your Dockerfile will look something like this:

FROM custom_elasticsearch_1

RUN elasticsearch-plugin install royrusso/elasticsearch-HQ

Inspired by @NickPridorozhko's answer, but updated and tested with elasticsearch^7.0.0 (with docker stack / swarm), example with analysis-icu:

elasticsearch:
  image: docker.elastic.co/elasticsearch/elasticsearch:7.3.0
  user: elasticsearch
  command: >
    /bin/sh -c "./bin/elasticsearch-plugin list | grep -q analysis-icu 
    || ./bin/elasticsearch-plugin install analysis-icu; 
    /usr/local/bin/docker-entrypoint.sh"
  ...

The main difference are the updated commands for ^7.0.0, and the use of the docker entrypoint instead of ./bin/elasticsearch (in a stack's context, you'd get an error related to a limit of spawnable processes).