Use an empty instance_name in docker compose example

[swarren12]

Description

Previously, the docker compose example used a default instance_name of "main", which meant that --remote_instance_name=main needed to be added to Bazel to get it to work as expected. This can be confusing for people who are just getting started and unfamiliar with all of the available configuration options. Since the value defaults to an empty string if this configuration item isn't provided, using that in the example should make it easier for someone to spin-up.

Fixes #1484

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Manually

Checklist

• [x] Updated documentation if needed (n/a)
• [x] Tests added/amended (n/a)
• [x] bazel test //... passes locally
• [x] PR is contained in a single commit, using git amend see some docs

---

This change is 

[CLAassistant]

All committers have signed the CLA.