docs: Move docker command up for clarity. (#9390)

## Description

Unburying the command for easier finding!

### Testing Instructions

👀

docs/repo-docs/guides/tools/docker.mdx

@@ -126,6 +126,12 @@ Splitting up **dependencies** and **source files** in this way lets us **only ru
 
 Our detailed [`with-docker` example](https://github.com/vercel/turborepo/tree/main/examples/with-docker) goes into depth on how to use `prune` to its full potential. Here's the Dockerfile, copied over for convenience.
 
+Build the Dockerfile from the root of your monorepo:
+
+```bash title="Terminal"
+docker build -f apps/web/Dockerfile .
+```
+
 <Callout type="info">
   This Dockerfile is written for a [Next.js](https://nextjs.org/) app that is
   using the `standalone` [output