Merge #1214

1214: Move services into core and optional r=mergify[bot] a=muhlemmer

## What type of PR?

Enhancement

## What does this PR do?

The current state of the services directory in the repo:
- Fetchmail -> Is actually **optional**
- Rspamd -> Postfix breaks without it, should be **core**
- Unbound -> Is actually **optional**

This PR moves those services into the proper core or optional directory. From now on we will have just 3 categories of images:
1. Core
2. Optional
3. Webmails

### Related issue(s)
- [Discussed](https://github.com/Mailu/Mailu/pull/1151#discussion_r320936937) in PR 1151

## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] In case of feature or enhancement: documentation updated accordingly **(I could not find any docs describing the repo layout. Please correct me if I was wrong)**
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.


Co-authored-by: Tim Möhlmann <muhlemmer@gmail.com>

tests/build.yml

@@ -8,7 +8,7 @@ services:
 
   resolver:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}unbound:${MAILU_VERSION:-local}
-    build: ../services/unbound
+    build: ../optional/unbound
 
   imap:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}dovecot:${MAILU_VERSION:-local}
@@ -20,7 +20,7 @@ services:
 
   antispam:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}rspamd:${MAILU_VERSION:-local}
-    build: ../services/rspamd
+    build: ../core/rspamd
 
   antivirus:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}clamav:${MAILU_VERSION:-local}
@@ -52,7 +52,7 @@ services:
 
   fetchmail:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}fetchmail:${MAILU_VERSION:-local}
-    build: ../services/fetchmail
+    build: ../optional/fetchmail
 
   none:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}none:${MAILU_VERSION:-local}

towncrier/newsfragments/1214.misc

@@ -0,0 +1 @@
+Move rspamd, fetchmail and unbound services into core and optional