docs(docker): add docker-compose.yml for one-command setup

Rewrite docker-compose.yml with a user-ready setup:
- pull_policy: always — keeps image up-to-date on every `up`
- network_mode: host — required for LAN/Tailscale streaming reach
- UNARR_API_KEY required variable with clear error message
- DOWNLOAD_DIR required variable
- named `unarr-data` volume for piece-DB + HLS cache (keeps them off NFS)
- macOS/Windows bridge + ports alternative in comments
- .env.example alongside with UNARR_API_KEY, DOWNLOAD_DIR, TZ

Quick start: cp .env.example .env && edit .env && docker compose up -d

.env.example

@@ -0,0 +1,16 @@
+# Copy this file to .env and fill in your values.
+# Then run: docker compose up -d
+
+# Your TorrentClaw API key (required).
+# Get it at: https://torrentclaw.com/settings/api-keys
+UNARR_API_KEY=tc_your_key_here
+
+# Absolute path to your media / downloads folder.
+# This is where finished movies and shows will be saved.
+DOWNLOAD_DIR=/home/youruser/Media
+
+# (Optional) Config directory — defaults to ./config next to this file.
+# CONFIG_DIR=/home/youruser/.config/unarr
+
+# (Optional) Timezone for logs.
+# TZ=Europe/Madrid