docs: clarify official Docker image tags

docker/README.md

@@ -1,19 +1,38 @@
 # Docker Images
 
-This directory contains Docker files for Mosquitto.
+This directory contains Dockerfiles for Eclipse Mosquitto.
 
-The `2.0` directory contains the latest version of Mosquitto for that
-series, it uses libressl. The `2.0-openssl` directory is identical except that
-it uses openssl instead of libressl, and enables TLS-PSK and TLS v1.3 cipher
-support.
+## Official image tags
 
-The `1.6` directory contains the version of Mosquitto based on the 1.6 branch.
-It uses libressl. The `1.6-openssl` directory is identical except that it uses
-openssl instead of libressl, and enables TLS-PSK support.
+Published Docker Official Images use the name `eclipse-mosquitto` on
+[Docker Hub](https://hub.docker.com/_/eclipse-mosquitto). The supported-tags
+list at the top of that page (and
+[library/eclipse-mosquitto](https://github.com/docker-library/official-images/blob/master/library/eclipse-mosquitto))
+is authoritative. As of the 2.1 line, those tags map as follows:
 
-The `1.5` directory contains the version of Mosquitto based on the 1.5 branch.
-It uses libressl. The `1.5-openssl` directory is identical except that it uses
-openssl instead of libressl, and enables TLS-PSK support.
+- **2.1.x (Alpine):** `2.1.2-alpine`, `2.1-alpine`, `alpine`, `2`, `latest`
+  — built from `2.1-alpine`. There is no separate non-Alpine `2.1` tag;
+  `2` and `latest` currently point at this Alpine 2.1 image.
+- **2.0.x (Alpine, OpenSSL):** `2.0.22`, `2.0.22-openssl`, `2.0`,
+  `2.0-openssl`, `2-openssl`, `openssl` — built from `2.0-openssl`.
+- **1.6.x (Alpine, OpenSSL):** `1.6.15-openssl`, `1.6-openssl` — built from
+  `1.6-openssl`.
+
+Official images have historically been Alpine-based. The `-openssl` tag
+suffix marks the older 2.0 / 1.6 OpenSSL builds (including TLS-PSK support).
+A Hub "Image Variants" section that mentions generic version / version-alpine
+patterns is shared Docker Official Images boilerplate and does not describe
+Mosquitto's tag layout; use the supported-tags list instead.
+
+## Dockerfiles in this repository
+
+- `2.1-alpine` — Mosquitto 2.1 on Alpine (OpenSSL). Source for the current
+  `latest` / `2` official tags.
+- `2.1-ubuntu` — Mosquitto 2.1 on Ubuntu. Not yet published to Docker
+  Official Images (architecture coverage differs from Alpine; see
+  [#3486](https://github.com/eclipse-mosquitto/mosquitto/issues/3486)).
+- `2.0-openssl` — Mosquitto 2.0 on Alpine with OpenSSL.
+- `1.6-openssl` — Mosquitto 1.6 on Alpine with OpenSSL.
 
 The `generic` directory contains a generic Dockerfile that can be used to build
 arbitrary versions of Mosquitto based on the released tarballs as follows: