Add pandoc microservice

[joecorall]

Adding new microservice:

1. Define the YML and Dockerfile
2. Update docker-compose.yml to deploy this service's docker image

    pandoc-dev: &pandoc
        <<: [*dev, *common]
        image: ${DOCKER_REPOSITORY}/scyllaridae-pandoc:pandoc
        networks:
            default:
                aliases:
                    - pandoc
    pandoc-prod: &pandoc-prod
        <<: [*prod, *pandoc]

3. Update alpaca.properties

derivative.pandoc.enabled=true
derivative.pandoc.in.stream=queue:islandora-connector-pandoc
derivative.pandoc.service.url=http://pandoc:8080/
derivative.pandoc.concurrent-consumers=-1
derivative.pandoc.max-concurrent-consumers=-1
derivative.pandoc.async-consumer=true

4. Add an action in Islandora
5. Add a context to trigger the action