Configure SERVER_URL for tat at runtime

[VenissaCarolQuadros]

Changes have been made to the Dockerfile and test-docker-compose.yml to:

1. Create an environment variable SERVER_URL in test-docker-compose (SERVER_NAME was not being used and has hence been removed)
2. Remove hardcoded server urls in Dockerfile. Instead command creates a config.js at run using the environment variable, and this is picked up by the application.

NOTE: Please merge this PR after Shared-Reality-Lab/IMAGE-TactileAuthoring#15 is merged.

TESTING:
Tested that the tat is able to update content to the correct server based on the environment variable value SERVER_URL using override (tested by swapping between prod and dev server urls). The version of tat on IMAGE-TactileAuthoring branch server_url was used for this. docker compose up -d tat --no-build was used to ensure that the image is not rebuilt after when these swaps were made.

Closes #1059

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