bugfix(.env.docker.example): Add .env.docker.example (#139)

.env.docker.example should be in repo since it is mentioned in README.md

Signed-off-by: Ariel Yu <ariel@ariels.xyz>

.env.docker.example

@@ -0,0 +1,52 @@
+# Copy to .env.docker and edit, then `docker compose up -d --build`:
+#   cp .env.docker.example .env.docker
+#
+# Loaded by docker-compose.yml (env_file). Commented lines fall back to the
+# image defaults (see docs/self-hosting.md for the full reference).
+
+# ===== Basics =====
+# Public URL behind a reverse proxy: password-reset links, the JMAP session
+# and inbound webhooks all use it.
+#APP_URL=https://mail.example.com
+
+# ===== Inbound mail: pick one (both can be on at once) =====
+# Built-in SMTP listener (default). Uncomment the "25:25" port mapping in
+# docker-compose.yml, point the domain's MX at this host, and set
+# MAIL_HOSTNAME to that host's name.
+#SMTP_INBOUND_PORT=25
+#MAIL_HOSTNAME=mail.example.com
+
+# Cloudflare Email Routing relay: keep MX on Cloudflare and deploy the Worker
+# in deploy/cloudflare-email-relay. Disables the built-in listener.
+#SMTP_INBOUND_PORT=0
+# Shared HMAC secret; must match the relay Worker's INBOUND_WEBHOOK_SECRET
+# (npx wrangler secret put INBOUND_WEBHOOK_SECRET).
+#INBOUND_WEBHOOK_SECRET=change-me-to-a-long-random-string
+
+# ===== Outbound mail: at least one =====
+# Any SMTP relay (host, Amazon SES, Postmark, Mailgun, your own Postfix):
+#SMTP_URL=smtps://user:pass@smtp.example.com:465
+
+# Cloudflare Email Sending: the zone must have Email Sending enabled.
+#CF_ACCOUNT_ID=
+#CF_TOKEN=
+
+# ===== Optional =====
+# Zone management: with zone + Email Routing permissions, adding a domain in
+# the app configures Email Routing and the sending subdomain automatically.
+#CF_TOKEN=
+
+# Resend / SES fallback credentials. Values saved on the domain page win.
+#RESEND_API_KEY=
+#AWS_ACCESS_KEY_ID=
+#AWS_SECRET_ACCESS_KEY=
+#AWS_REGION=
+
+# Bot protection on login and reset forms (site key at build time via
+# NEXT_PUBLIC_TURNSTILE_SITE_KEY).
+#TURNSTILE_SECRET_KEY=
+
+# Email assistant (OpenAI-compatible API).
+#AI_BASE_URL=
+#AI_API_KEY=
+#AI_MODEL=gpt-4o-mini