docs(docker): add compose environment example

[RobinALG87]

Summary

Add a small .env.example for Docker Compose users so they can customize the published port and PostgreSQL settings without editing docker-compose.yml.

Changes

• Add .env.example with the current Docker Compose defaults.
• Read Docker Compose port and PostgreSQL values from environment variables with the same fallback defaults.
• Ignore local .env files and document the copy/edit workflow in the README.

Testing

• Ran git diff --check upstream/master...origin/docs/docker-compose-env-example.
• Ran docker compose config with default values.
• Ran PowerShell override check with HAPI_FHIR_PORT=8888 plus custom POSTGRES_DB, POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_HOST, and POSTGRES_PORT, then ran docker compose config.

[Copilot]

Pull request overview

This PR improves the Docker Compose developer experience by introducing an .env.example file and switching docker-compose.yml to use environment-variable overrides, so users can customize the published HTTP port and PostgreSQL settings without editing tracked YAML.

Changes:

• Add .env.example containing the current Docker Compose default values.
• Parameterize the JPA server port mapping and PostgreSQL connection settings via ${VAR:-default} interpolation in docker-compose.yml.
• Update docs to describe the .env.example → .env copy/edit workflow and ignore local .env files via .gitignore.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
README.md	Documents the new .env.example workflow for overriding Compose defaults (port + PostgreSQL settings).
docker-compose.yml	Reads port and PostgreSQL config from environment variables with fallback defaults.
.gitignore	Ignores .env to avoid committing local Compose overrides.
.env.example	Provides a copyable baseline of the Compose defaults for user overrides.

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