Introduce a DKIM_SELECTOR environment variable for controlling dkim_selector

Fixes #22

Author: spantaleev

Dockerfile

@@ -24,6 +24,7 @@ ENV LOCAL_DOMAINS=@ \
     RELAY_TO_DOMAINS=* \
     RELAY_TO_USERS= \
     DISABLE_SENDER_VERIFICATION= \
+    DKIM_SELECTOR=default \
     HOSTNAME= \
     SMARTHOST= \
     SMTP_PASSWORD= \

README.md

@@ -71,6 +71,8 @@ docker run \
        ghcr.io/devture/exim-relay:SOME_TAGGED_RELEASE
 ```
 
+You can adjust the [DKIM selector](https://dmarcly.com/blog/what-is-dkim-selector-and-how-does-it-work-dkim-selector-explained) value by passing an additional `DKIM_SELECTOR` environment variable. By default, it has a value of `default`.
+
 ## Docker Compose
 
 ```

exim.conf

@@ -757,7 +757,7 @@ remote_smtp:
   headers_remove = received
   helo_data = ${env{HOSTNAME}{$value}{$primary_hostname}}
   dkim_domain = ${env{HOSTNAME}{$value}{$primary_hostname}}
-  dkim_selector = default
+  dkim_selector = ${env{DKIM_SELECTOR}{$value}{default}}
   # Headers as per recommended signature content of the RFC 4871: https://datatracker.ietf.org/doc/html/rfc4871#section-5.5
   dkim_sign_headers = From:=Sender:=Reply-To:=Subject:=Date:=Message-ID:=To:=Cc:=MIME-Version:=Content-Type:=Content-Transfer-Encoding:=Content-ID:=Content-Description:=In-Reply-To:=References:=List-Id:=List-Help:=List-Unsubscribe:=List-Subscribe:=List-Post:=List-Owner:=List-Archive
   dkim_private_key = ${if exists{/etc/exim/dkim.pem} {/etc/exim/dkim.pem}}