docs(learn): link Docker install tab to env var configuration reference (#643)

* docs(learn): link Docker install tab to env var configuration reference

The Docker tab in the install guide mentioned DEFAULTS_MODE,
REPLICATION_HOSTNAME, and "preset environment variables" without ever
pointing at the canonical reference, so readers never learned that any
Harper configuration option can be set through an environment variable.

Add a short tip inside the Docker tab that states the general rule, gives
the SCREAMING_SNAKE_CASE mapping in one line, and links to the
Environment Variables section of the configuration reference.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* docs(learn): fix subject-verb agreement in Docker env var tip

Review feedback on #643: "makes `-e` flags the natural way" mismatched
plural subject with singular complement. Gerund "using" fixes it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* docs(learn): scope Docker env var tip to instance configuration

The tip claimed *any* Harper configuration option can be set with an
environment variable, which a Docker user could reasonably apply to a
component's config.yaml, where it silently does nothing. Scope the claim
to harper-config.yaml and state the component-configuration exception
using the same wording as reference/configuration/overview.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: Ethan-Arrowood

learn/getting-started/install-and-connect-harper.mdx

@@ -110,6 +110,10 @@ The image is configured to automatically run the `harper` command upon startup.
 
 The Harper installation process is normally an interactive prompt; however, it also supports overrides using environment variables or CLI arguments. Since the image contains preset environment variables and additional environment variables `DEFAULTS_MODE` and `REPLICATION_HOSTNAME` are included in the `docker run` command, Harper will complete its installation step completely non-interactively.
 
+:::tip
+This is not limited to installation: any option in Harper's own `harper-config.yaml` can be set with an environment variable by mapping its YAML key to `SCREAMING_SNAKE_CASE` (`http.port` becomes `HTTP_PORT`, `replication.hostname` becomes `REPLICATION_HOSTNAME`), which makes using `-e` flags the natural way to configure a containerized instance. This applies to instance configuration only — component configuration, the settings in a component's own `config.yaml`, cannot be set via environment variables or CLI arguments. See [Environment Variables](/reference/v5/configuration/overview#2-environment-variables) in the configuration reference for the full naming convention.
+:::
+
   </TabItem>
 </Tabs>