Add docs for how to create a smtp gateway

tools/exim/README.md

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+# SMTP Gateway via Docker
+
+This is an example setup for a SMTP server that forwards all incoming
+mails to docspell via `curl`.
+
+The docker image contains [exim](https://exim.org) and a sample config
+file that runs curl against a configurable docspell url. It uses the
+[integration
+endpoint](https://docspell.org/doc/uploading#integration-endpoint) and
+it expects it to be configured with "http-header" protection. It can
+be easily adopted to use a different protection method.
+
+Please see the [documentation
+page](https://docspell.org/doc/tools/smtpgateway) for a guide.

tools/exim/docker-compose.yml

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+version: '3.7'
+services:
+  smtp:
+    image: ds-exim:latest
+    ports:
+      - "25:25"
+    environment:
+      # This is the configured header value for the integration-endpoint
+      - DS_HEADER=test123
+      # This is the URL to docspell
+      - DS_URL=http://192.168.1.95:7880
+    # Use host network for demo purposes
+    network_mode: host

tools/exim/exim.conf

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+## Provide certificates to enable StartTLS
+# tls_certificate = /var/lib/acme/test.org/fullchain.pem
+# tls_privatekey = /var/lib/acme/test.org/key.pem
+tls_advertise_hosts =
+
+primary_hostname = test.org
+domainlist local_domains = test.org
+timeout_frozen_after = 1m
+acl_smtp_rcpt = acl_check_rcpt
+acl_smtp_data = acl_check_data
+never_users = root
+host_lookup = *
+daemon_smtp_ports = 25
+
+message_size_limit = 30m
+
+keep_environment = DS_HEADER : DS_URL
+
+begin acl
+acl_check_rcpt:
+require
+  domains = +local_domains
+require
+  message = Sender verification failed
+  verify = sender
+require
+  message = Receiver verification failed
+  verify = recipient
+require
+  message = Recipient unknown
+  condition = ${run{/usr/bin/curl --out /dev/null --silent --fail -H "Docspell-Integration: ${env{DS_HEADER}{$value} fail}" "${env{DS_URL}{$value} fail}/api/v1/open/integration/item/$local_part"}{yes}{no}}
+warn
+  message = Reverse lookup failed
+  !verify = reverse_host_lookup
+accept
+
+acl_check_data:
+deny
+  message = Sender verification failed
+  !verify = header_sender
+accept
+
+begin routers
+local_users:
+  driver = accept
+  transport = docspell
+
+begin transports
+docspell:
+  driver = pipe
+  command = /usr/bin/curl --out /dev/null --silent --fail -H "Docspell-Integration: ${env{DS_HEADER}{$value} fail}" -F "file=@-;filename=\"$h_subject:\"" "${env{DS_URL}{$value} fail}/api/v1/open/integration/item/$local_part"
+  return_fail_output
+  user = nobody
+  delivery_date_add
+  envelope_to_add
+  return_path_add
+  log_output

tools/exim/exim.dockerfile

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+FROM alpine:latest
+
+RUN apk add --no-cache exim curl
+USER exim
+COPY ./exim.conf /etc/exim/
+EXPOSE 25
+ENTRYPOINT ["exim"]
+CMD ["-bdf", "-v", "-q1m"]