refactor: Share a common helper (vhost builder) for sourcing domains (#2620)

* chore: Split vhost helper method and use filepath vars

- Helpers `accounts.sh` and `aliases.sh` can move their vhost code into this helper.
- They share duplicate code with `bin/open-dkim` which will also leverage this vhost helper going forward.

* chore: Sync vhost generation logic into helper

- Chunky commit, but mostly copy/paste of logic into a common method.
- `bin/open-dkim` additionally wrapped relevant logic in a function call and revised inline docs.

* chore: Include LDAP vhost support

- Revises notes for LDAP vhost support.
- This now ensures LDAP users get vhost rebuilt to match the startup script for when change detection support is enabled.
- `bin/open-dkim` will additionally be able to support the default `DOMAINNAME` var (set via `helpers/dns.sh`) for LDAP users instead of requiring them to provide one explicitly.

* chore(`bin/open-dkim`): Ensure `DOMAINNAME` is properly set

- This will ensure LDAP users insert the same `DOMAINNAME` value as used during container startup.
- The container itself should panic at startup (during `helpers/dns.sh`) if this isn't configured correctly already, thus it should not introduce any breaking change to users of this utility?

* chore: Set the 2nd value as blank `_`

Line is split by a delimiter such as white-space (or via IFS: `|`), the blank `_` var is to indicate we're not interested in that value, but still leverage how `read -r` works, instead of splitting the var ourselves first thing.

* chore: Remove shellcheck disable lines

No longer applicable with the switch to `_`

target/bin/open-dkim

@@ -98,43 +98,28 @@ do
   esac
 done
 
-DATABASE_ACCOUNTS='/tmp/docker-mailserver/postfix-accounts.cf'
-DATABASE_VIRTUAL='/tmp/docker-mailserver/postfix-virtual.cf'
-DATABASE_VHOST='/tmp/vhost'
-TMP_VHOST='/tmp/vhost.dkim.tmp'
-touch "${TMP_VHOST}"
-if [[ -z ${DOMAINS} ]]
-then
-  # getting domains FROM mail accounts
-  if [[ -f ${DATABASE_ACCOUNTS} ]]
+DATABASE_VHOST='/tmp/vhost.dkim'
+# Prepare a file with one domain per line:
+function _generate_domains_config
+{
+  local TMP_VHOST='/tmp/vhost.dkim.tmp'
+
+  # Generate the default vhost (equivalent to /etc/postfix/vhost),
+  # unless CLI arg DOMAINS provided an alternative list to use instead:
+  if [[ -z ${DOMAINS} ]]
   then
-    # shellcheck disable=SC2034
-    while IFS=$'|' read -r LOGIN PASS
-    do
-      DOMAIN=$(echo "${LOGIN}" | cut -d @ -f2)
-      echo "${DOMAIN}" >>"${TMP_VHOST}"
-    done < <(_get_valid_lines_from_file "${DATABASE_ACCOUNTS}")
+    _obtain_hostname_and_domainname
+    # uses TMP_VHOST:
+    _vhost_collect_postfix_domains
+  else
+    tr ',' '\n' <<< "${DOMAINS}" >"${TMP_VHOST}"
   fi
 
-  # getting domains FROM mail aliases
-  if [[ -f ${DATABASE_VIRTUAL} ]]
-  then
-    # shellcheck disable=SC2034
-    while read -r FROM TO
-    do
-      UNAME=$(echo "${FROM}" | cut -d @ -f1)
-      DOMAIN=$(echo "${FROM}" | cut -d @ -f2)
-
-      [[ ${UNAME} != "${DOMAIN}" ]] && echo "${DOMAIN}" >>"${TMP_VHOST}"
-    done < <(_get_valid_lines_from_file "${DATABASE_VIRTUAL}")
-  fi
-else
-  tr ',' '\n' <<< "${DOMAINS}" >"${TMP_VHOST}"
-fi
-
-sort < "${TMP_VHOST}" | uniq >"${DATABASE_VHOST}"
-rm "${TMP_VHOST}"
+  # uses DATABASE_VHOST + TMP_VHOST:
+  _create_vhost
+}
 
+_generate_domains_config
 if [[ ! -s ${DATABASE_VHOST} ]]
 then
   _log 'warn' 'No entries found, no keys to make'