Fix docker context (#330)

### Description

This pull request fixes a build failure caused by an incorrect context
in a Docker Compose configuration file.
Additionally, it slightly expands the Docker instructions.

### Changes

- Corrected the context in a Docker Compose file to fix the build issue.
- Expanded the Docker instructions.

docs/docker.md

@@ -11,4 +11,15 @@ All of the examples in this repo provide instructions for being run normally, an
 
 If starting from scratch and your host machine is Ubuntu 22.04, you can use accomplish all of the above with the convenience scripts `scripts/docker/install_docker_ubuntu22.sh` and `scripts/docker/install_nvidia_container_toolkit.sh`.
 
+Build the Docker image and start the container with the following command:
+```bash
+docker compose -f scripts/docker/compose.yml up --build
+```
+
+To build and run the Docker image for a specific example, use the following command:
+```bash
+docker compose -f examples/<example_name>/compose.yml up --build
+```
+where `<example_name>` is the name of the example you want to run.
+
 During the first run of any example, Docker will build the images. Go grab a coffee while this happens. Subsequent runs will be faster since the images are cached.

scripts/docker/compose.yml

@@ -1,10 +1,10 @@
 # Run with:
-# docker compose -f scripts/compose.yml up --build
+# docker compose -f scripts/docker/compose.yml up --build
 services:
   openpi_server:
     image: openpi_server
     build:
-      context: ..
+      context: ../..
       dockerfile: scripts/docker/serve_policy.Dockerfile
     init: true
     tty: true