fix: update-check.sh should query GH Releases (#3666)

* fix: Source `VERSION` from image ENV

Now CI builds triggered from tagged releases will always have the correct version. No need for manually updating a separate file.

* fix: Query latest GH release tag

Compare to the remote GH release tag published, rather than contents of a `VERSION` file.

`VERSION` file remains in source for now as prior releases still rely on it for an update notification.

* chore: Switch from `yq` to `jaq`

- Can more easily express a string subslice.
- Lighter weight: 9.3M vs 1.7M.
- Drawback, no YAML input/output support.

If `yq` is preferred, the `v` prefix could be removed via BASH easily enough.

* chore: Add entry to `CHANGELOG.md`

* ci: `VERSION` has no relevance to `:edge`

* docs: Update build guide + simplify `make build`

---------

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

target/scripts/update-check.sh

@@ -3,8 +3,8 @@
 # shellcheck source=./helpers/log.sh
 source /usr/local/bin/helpers/log.sh
 
-VERSION=$(</VERSION)
-VERSION_URL='https://raw.githubusercontent.com/docker-mailserver/docker-mailserver/master/VERSION'
+VERSION="${DMS_RELEASE#v}"
+VERSION_URL='https://github.com/docker-mailserver/docker-mailserver/releases/latest'
 CHANGELOG_URL='https://github.com/docker-mailserver/docker-mailserver/blob/master/CHANGELOG.md'
 
 # check for correct syntax
@@ -17,7 +17,8 @@ fi
 
 while true; do
   # get remote version information
-  LATEST=$(curl -Lsf "${VERSION_URL}")
+  # JSON response provides a field for the release tag, the `v` prefix is removed with `[1:]`
+  LATEST=$(curl -sfL -H 'accept: application/json' "${VERSION_URL}" | jaq -r '.tag_name[1:]')
 
   # did we get a valid response?
   if [[ ${LATEST} =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]; then