Don't unconditionally override ENTRYPOINT

Hello,

Overriding ENTRYPOINT unconditionnaly blocks certain usages, notably using antora in gitlab-ci to generate documentation. At the moment we need to disable antora docker image entry point in our pipelines to let gitlab runner do its magic before actually running antora, e.g. :

image:
  name: antora/antora:2.0.0
  entrypoint: []
doc:generate:
  script:
    - antora generate site.yml

Two possible fixes :

1. Use CMD instead of ENTRYPOINT, unfortunately you break compatibility for actual users
2. Use a helper script as does the base image node, ref: https://github.com/nodejs/docker-node/pull/1043

I'll try to push a MR with solution 2 this evening.

Regards, Nicolas