fix non-copyable code blocks in “Use Chisel in a Dockerfile

# Background

Chisel is a developer tool for extracting highly customized and specialized package slices of Ubuntu packages to create compact, secure software.

The Chisel documentation includes a “[Use Chisel in a Dockerfile](https://ubuntu.com/chisel/docs/latest/how-to/use-chisel-in-dockerfile/)” page with code blocks that are difficult to copy cleanly. This creates friction for readers trying to follow the guide step by step and reproduce the example commands or snippets in their own environment.

This is a duplicate from canonical/open-documentation-academy#381

# Task

Fix the code blocks in “Use Chisel in a Dockerfile” so they can be copied reliably and remain easy to read in the rendered documentation.

Suggested steps:
* Review the page and identify which code blocks are hard to copy cleanly.
* Update the formatting so copied commands and snippets do not include unwanted prompt characters, line-wrapping artifacts, or other formatting problems.
* Check the rendered page to confirm the revised code blocks are still readable and consistent with the surrounding documentation.

Once you’re assigned this issue, please open a pull request directly into [canonical/chisel-docs](https://github.com/canonical/chisel-docs). In order to contribute, you must sign the [Canonical contributor license agreement](https://ubuntu.com/legal/contributors).

# What you will learn

* Improving usability in tutorial documentation.
* Formatting code examples for copyability and readability.
* Reviewing rendered documentation behavior.

# Outcome

The code blocks in “Use Chisel in a Dockerfile” can be copied cleanly by readers and remain correctly formatted in the documentation.

# Resources

* [Chisel repository](https://github.com/canonical/chisel-docs)
* [Use Chisel in a Dockerfile page](https://github.com/canonical/chisel-docs/blob/main/docs/how-to/use-chisel-in-dockerfile.md)

# Mentor

The mentor for this issue is @asanvaq. They are the Technical Author for Rockcraft and Chisel.

Please feel free to reach out to the mentor on [Matrix](https://matrix.to/#/@sanchovaquera:ubuntu.com) if you have further questions.