docs: elaborate on Docker tag

Also link to installation from README.md

Closes PostgREST#4385.

Author: steve-chavez

README.md

@@ -52,8 +52,8 @@ Big thanks to our sponsors! You can join them by supporting PostgREST on [Patreo
 
 ## Usage
 
-1. Download the binary ([latest release](https://github.com/PostgREST/postgrest/releases/latest))
-   for your platform.
+1. See the docs for [how to install PostgREST on your platform](https://docs.postgrest.org/en/stable/explanations/install.html). You can also [use Docker](https://docs.postgrest.org/en/stable/explanations/install.html#docker).
+
 2. Invoke for help:
 
     ```bash

docs/explanations/install.rst

@@ -75,8 +75,12 @@ You can get the `official PostgREST Docker image <https://hub.docker.com/r/postg
 
 .. code-block:: bash
 
+  # pull the latest version
   docker pull postgrest/postgrest
 
+  # to pull a particular version, use one of the versions on https://hub.docker.com/r/postgrest/postgrest/tags
+  docker pull postgrest/postgrest:<version>
+
 To configure the container image, use :ref:`env_variables_config`.
 
 There are two ways to run the PostgREST container: with an existing external database, or through docker-compose.