feat: Enable customizing the docker admin password

[c-w]

SUMMARY

This PR modifies the docker-compose setup to enable customizing the password used for the admin user. This is useful in scenarios where we want to use the docker-compose setup to spin up a self-contained superset instance but when we don't want to risk someone being able to log in as an admin with the known default credentials.

TESTING INSTRUCTIONS

1. Configure password override: echo 'ADMIN_PASSWORD=supersecret' > docker/.env-local
2. Run the stack: TAG=4.0.2 docker compose -f docker-compose-image-tag.yml up
3. Go to http://localhost:8088 and log in using username admin and the password from step 1

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[github-actions]

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[mistercrunch]

Thanks for the contribution!