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>

docs/content/examples/tutorials/docker-build.md

@@ -10,7 +10,7 @@ You'll need to retrieve the git submodules prior to building your own Docker ima
 
 ```sh
 git submodule update --init --recursive
-docker build -t <YOUR CUSTOM IMAGE NAME> .
+docker build --tag <YOUR CUSTOM IMAGE NAME> .
 ```
 
 Or, you can clone and retrieve the submodules in one command:
@@ -21,19 +21,26 @@ git clone --recurse-submodules https://github.com/docker-mailserver/docker-mails
 
 ### About Docker
 
-#### Version
+#### Minimum supported version
 
-We make use of build-features that require a recent version of Docker. Depending on your distribution, please have a look at [the official installation documentation for Docker](https://docs.docker.com/engine/install/) to get the latest version. Otherwise, you may encounter issues, for example with the `--link` flag for a [`#!dockerfile COPY`](https://docs.docker.com/engine/reference/builder/#copy) command.
+We make use of build features that require a recent version of Docker. v23.0 or newer is advised, but earlier releases may work.
 
-#### Environment
+- To get the latest version for your distribution, please have a look at [the official installation documentation for Docker](https://docs.docker.com/engine/install/).
+- If you are using a version of Docker prior to v23.0, you will need to enable BuildKit via the ENV [`DOCKER_BUILDKIT=1`](https://docs.docker.com/build/buildkit/#getting-started).
 
-If you are not using `make` to build the image, note that you will need to provide `DOCKER_BUILDKIT=1` to the `docker build` command for the build to succeed.
+#### Build Arguments (Optional)
 
-#### Build Arguments
+The `Dockerfile` includes several build [`ARG`][docker-docs::builder-arg] instructions that can be configured:
 
-The `Dockerfile` takes additional, so-called build arguments. These are
+- `DOVECOT_COMMUNITY_REPO`: Install Dovecot from the community repo instead of from Debian (default = 1) 
+- `DMS_RELEASE`: The image version (default = edge)
+- `VCS_REVISION`: The git commit hash used for the build (default = unknown)
 
-1. `VCS_VERSION`: the image version (default = edge)
-2. `VCS_REVISION`: the image revision (default = unknown)
+!!! note
 
-When using `make` to build the image, these are filled with proper values. You can build the image without supplying these arguments just fine though.
+    - `DMS_RELEASE` (_when not `edge`_) will be used to check for updates from our GH releases page at runtime due to the default feature [`ENABLE_UPDATE_CHECK=1`][docs::env-update-check].
+    - Both `DMS_RELEASE` and `VCS_REVISION` are also used with `opencontainers` metadata [`LABEL`][docker-docs::builder-label] instructions.
+
+[docs::env-update-check]: https://docker-mailserver.github.io/docker-mailserver/latest/config/environment/#enable_update_check
+[docker-docs::builder-arg]: https://docs.docker.com/engine/reference/builder/#using-arg-variables
+[docker-docs::builder-label]: https://docs.docker.com/engine/reference/builder/#label