Expose the actual GUI ports (8080/8181) and the content server port (8081)

[azullus]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Adds EXPOSE 8080 8181 8081 to Dockerfile and Dockerfile.aarch64 (there is no armhf Dockerfile), using the same # ports and volumes convention as docker-calibre-web.

Benefits of this PR and context:

docker image inspect lscr.io/linuxserver/calibre --format '{{json .Config.ExposedPorts}}' returns {"3000/tcp":{},"3001/tcp":{}}, which are inherited from baseimage-selkies. This image moves the Selkies nginx frontend to 8080/8181 via CUSTOM_PORT / CUSTOM_HTTPS_PORT, so nothing listens on 3000/3001 (ss -tlnp in a running container shows nginx on 8080 and 8181).

Tools that read ExposedPorts pick the wrong port: Traefik's automatic port detection, Portainer/Unraid templates, compose validators and healthcheck generators. The result is connection refused on 3000 and an unhealthy container. We lost a deploy cycle to this before checking the live sockets.

This PR adds EXPOSE 8080 8181 8081 to both Dockerfiles, matching the ports in readme-vars.yml (8081 = the optional Calibre content server). Docker can't remove an inherited EXPOSE, so 3000/3001 will still appear, but the real ports will now be listed as well.

• Changes replicated in Dockerfile.aarch64
• No readme or changelog change needed. The ports are already documented. Add a readme-vars.yml changelog line if maintainers want one.

How Has This Been Tested?

Checked against a running lscr.io/linuxserver/calibre container: docker image inspect --format '{{json .Config.ExposedPorts}}' shows only 3000/tcp and 3001/tcp, while ss -tlnp inside the container shows nginx listening on 8080 and 8181. The change is metadata-only (EXPOSE does not publish or bind anything), so runtime behaviour is unchanged.

Source / References:

• docker-baseimage-selkies Dockerfile: EXPOSE 3000 3001
• This repo's Dockerfile: CUSTOM_PORT="8080", CUSTOM_HTTPS_PORT="8181"
• readme-vars.yml documents 8080 / 8181 / 8081