readme: Add example for running just one integration container

[ajgajg1134]

What does this PR do?

Adds a useful example for how to run a single container from our docker-compose

Motivation

This came in handy recently when I wanted to run just a single mysql unit test and didn't want to download / deal with the overhead of running every single container listed in our docker-compose

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-13 19:17:14

Comparing candidate commit 1a2aed1 in PR branch andrew.glaude/add-docker-example with baseline commit 417a56e in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.