Document SYNBIOHUB_BACKEND environment variable configuration

[Copilot]

Adds documentation for the SYNBIOHUB_BACKEND environment variable introduced in synbiohub2-docker to configure the frontend's backend URL.

Changes

• Configuration section in installation.md: Documents SYNBIOHUB_BACKEND environment variable with default value http://localhost:7777
• Two configuration methods: Command-line environment variable and .env file approach
• Production context: Notes relevance for deployments where backend runs on different server/port

# Method 1: Command-line
SYNBIOHUB_BACKEND=http://your-backend-url:port docker-compose -f ./docker-compose.yml up

# Method 2: .env file
cp .env.example .env
# Edit .env and uncomment/set SYNBIOHUB_BACKEND
docker-compose -f ./docker-compose.yml up

Section placed after base installation steps, before SBOLExplorer configuration to maintain logical flow.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update info for setting your backend url</issue_title>
<issue_description>following the readme changes in this pull request https://github.com/SynBioHub/synbiohub2-docker/pull/25/changes
we need to now explain how to configure your backend url in the docs as well</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update info for setting your backend url #8

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[PeterHindes]

@copilot verify that the two methods of setting work by changing the port number and testing in your environment @copilot

[Copilot]

@copilot verify that the two methods of setting work by changing the port number and testing in your environment @copilot

I've verified both configuration methods work correctly by testing with the synbiohub2-docker repository (commit e80f233):

Test Results:

1. ✅ Default behavior: http://localhost:7777 (when no env var set)
2. ✅ Command-line method: Changed port to 8888 successfully

   SYNBIOHUB_BACKEND=http://localhost:8888 docker compose config
   # Result: backend: http://localhost:8888

3. ✅ .env file method: Changed port to 9999 successfully

   cp .env.example .env
   # Edit .env: SYNBIOHUB_BACKEND=http://localhost:9999
   docker compose config
   # Result: backend: http://localhost:9999

The docker-compose.yml uses ${SYNBIOHUB_BACKEND:-http://localhost:7777} syntax, which correctly reads the environment variable or falls back to the default. Both documented methods are working as expected.