Fix healthcheck to not use env variable

[yannis-srl]

Description

Changing the healthcheck in the example docker compose to not use the environment variable which only specifies the exposed port.

Motivation and Context

This PR addresses: #862

This makes it possible to change the ports by setting the env variables, without bloodhound breaking.

How Has This Been Tested?

I have run it to avoid syntax errors by mistyping.

Screenshots (optional):

Types of changes

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

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR (I do not have the permissions to do this. I have done all other steps.)
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https://github.com/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

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[yannis-srl]

I have read the CLA Document and I hereby sign the CLA