Making system executable available to a Github Action - ruby

Question

My Github action is calling my build.rb script which depends on the pandoc executable being available to the system (this is actually done by a dependency, so I can't change it).
I added pandoc to the Github action as per the instructions here https://github.com/pandoc/pandoc-action-example but my build.rb script is still erroring with sh: 1: pandoc: not found.
I think maybe I have to add the docker-compiled pandoc to the PATH so it can be used by my script, but I'm not sure how to do that.
This is the result from the uses: docker://pandoc/core:2.9 step:
Run docker://pandoc/core:2.9
/usr/bin/docker run --name pandoccore29_2dccd6 --label 372a9e --workdir /github/workspace --rm -e HOME -e GITHUB_JOB -e GITHUB_REF -e GITHUB_SHA -e GITHUB_REPOSITORY -e GITHUB_REPOSITORY_OWNER -e GITHUB_RUN_ID -e GITHUB_RUN_NUMBER -e GITHUB_RETENTION_DAYS -e GITHUB_RUN_ATTEMPT -e GITHUB_ACTOR -e GITHUB_WORKFLOW -e GITHUB_HEAD_REF -e GITHUB_BASE_REF -e GITHUB_EVENT_NAME -e GITHUB_SERVER_URL -e GITHUB_API_URL -e GITHUB_GRAPHQL_URL -e GITHUB_REF_NAME -e GITHUB_REF_PROTECTED -e GITHUB_REF_TYPE -e GITHUB_WORKSPACE -e GITHUB_ACTION -e GITHUB_EVENT_PATH -e GITHUB_ACTION_REPOSITORY -e GITHUB_ACTION_REF -e GITHUB_PATH -e GITHUB_ENV -e GITHUB_STEP_SUMMARY -e RUNNER_OS -e RUNNER_ARCH -e RUNNER_NAME -e RUNNER_TOOL_CACHE -e RUNNER_TEMP -e RUNNER_WORKSPACE -e ACTIONS_RUNTIME_URL -e ACTIONS_RUNTIME_TOKEN -e ACTIONS_CACHE_URL -e GITHUB_ACTIONS=true -e CI=true -v "/var/run/docker.sock":"/var/run/docker.sock" -v "/home/runner/work/_temp/_github_home":"/github/home" -v "/home/runner/work/_temp/_github_workflow":"/github/workflow" -v "/home/runner/work/_temp/_runner_file_commands":"/github/file_commands" -v "/home/runner/work/maxpleaner.github.io/maxpleaner.github.io":"/github/workspace" pandoc/core:2.9
Github action is shown below.
name: Ruby
on:
push:
branches: [ master ]
permissions:
contents: read
jobs:
build:
runs-on: ubuntu-latest
strategy:
matrix:
ruby-version: ['3.0']
steps:
- uses: actions/checkout#v3
- name: Setup Ruby
uses: ruby/setup-ruby#v1
with:
ruby-version: ${{ matrix.ruby-version }}
bundler-cache: true # runs 'bundle install' and caches installed gems automatically
- uses: docker://pandoc/core:2.9
- name: Build
run: bundle exec ruby build.rb
- name: Deploy
uses: s0/git-publish-subdir-action#develop
env:
REPO: self
BRANCH: gh-pages # The branch name where you want to push the assets
FOLDER: dist # The directory where your assets are generated
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # GitHub will automatically add this - you don't need to bother getting a token
MESSAGE: "Build: ({sha}) {msg}" # The commit message