docs: Dockerfile - Use ref links

Dockerfile

@@ -16,7 +16,7 @@ RUN <<HEREDOC
   # Install packages to a custom root-fs location (defined in `ZYPPER_OPTIONS`):
   zypper "${ZYPPER_OPTIONS[@]}" --gpg-auto-import-keys refresh
   zypper "${ZYPPER_OPTIONS[@]}" --non-interactive install --download-in-advance --no-recommends \
-       bash procps grep gawk sed coreutils busybox ldns libidn2-0 socat openssl curl
+    bash procps grep gawk sed coreutils busybox ldns libidn2-0 socat openssl curl
 
   # Optional - Avoid `CACHE_ZYPPER` from being redundantly cached in this RUN layer:
   # (doesn't improve `INSTALL_ROOT` size thanks to `--cache-dir`)

Dockerfile.md

@@ -19,19 +19,19 @@ docker run --rm -it -v /tmp:/data --workdir /data ghcr.io/testssl/testssl.sh:3.2
 
 > [!NOTE]
 > - The UID/GID ownership of the file will be created by the container user `testssl` (`1000:1000`), with permissions `644`.
-> - Your host directory must permit the `testssl` container user or group to write to that host volume. You could alternatively use [`docker cp`](https://docs.docker.com/reference/cli/docker/container/cp/).
+> - Your host directory must permit the `testssl` container user or group to write to that host volume. You could alternatively use [`docker cp`][docker-docs::cli::cp].
 
 ## Images
 
 ### Available at DockerHub and GHCR
 
 You can pull the image from either of these registries:
-- DockerHub: [`drwetter/testssl.sh`](https://hub.docker.com/r/drwetter/testssl.sh)
+- DockerHub: [`drwetter/testssl.sh`][image-registry::dockerhub]
-- GHCR: [`ghcr.io/testssl/testssl.sh`](https://github.com/testssl/testssl.sh/pkgs/container/testssl.sh)
+- GHCR: [`ghcr.io/testssl/testssl.sh`][image-registry::ghcr]
 
 Supported tags:
 - `3.2` / `latest`
-- `3.0` is the old stable version ([soon to become EOL](https://github.com/testssl/testssl.sh/tree/3.0#status))
+- `3.0` is the old stable version ([soon to become EOL][testssl::v3p0-eol])
 
 ### Building the `testssl.sh` container image
 
@@ -47,7 +47,9 @@ There are two base images supported:
 - openSUSE Leap ([`Dockerfile`](./Dockerfile)), glibc-based + faster.
 - Alpine ([`Dockerfile`](./Dockerfile.alpine)), musl-based + half the size.
 
-The Alpine variant is made available if you need broader platform support, or an image about 30MB smaller at the expense of slightly slower performance.
+The Alpine variant is made available if you need broader platform support, or an image about 30MB smaller at the expense of [slightly slower performance][testssl::base-image-performance].
+
+For contributors, if needing context on the [package selection has been documented][testssl::base-image-packages] for each base image.
 
 #### Tip - Remote build context + `Dockerfile`
 
@@ -58,7 +60,7 @@ docker build --tag localhost/testssl.sh:3.2 https://github.com/testssl/testssl.s
 ```
 
 > [!NOTE]
-> This will produce a slightly larger image as [`.dockerignore` is not supported with remote build contexts](https://github.com/docker/buildx/issues/3169).
+> This will produce a slightly larger image as [`.dockerignore` is not supported with remote build contexts][build::dockerignore-remote-context].
 
 ---
 
@@ -70,3 +72,11 @@ docker build \
   --file https://raw.githubusercontent.com/testssl/testssl.sh/3.2/Dockerfile.alpine \
   https://github.com/testssl/testssl.sh.git#3.2
 ```
+
+[docker-docs::cli::cp]: https://docs.docker.com/reference/cli/docker/container/cp/
+[image-registry::dockerhub]: https://hub.docker.com/r/drwetter/testssl.sh
+[image-registry::ghcr]: https://github.com/testssl/testssl.sh/pkgs/container/testssl.sh
+[testssl::v3p0-eol]: https://github.com/testssl/testssl.sh/tree/3.0#status
+[testssl::base-image-performance]: https://github.com/testssl/testssl.sh/issues/2422#issuecomment-2841822406
+[testssl::base-image-packages]: https://github.com/testssl/testssl.sh/issues/2422#issuecomment-2841822406
+[build::dockerignore-remote-context]: https://github.com/docker/buildx/issues/3169