add mock stripe key to docker

[ItsEricSun]

Description

Added mock stripe key to docker compose since was getting this error:

eric@EYoga:~/fcc$ docker logs fcc-backend-dev-1

> @scaffolding/source@0.0.0 start:dev:docker
> npx nodemon --watch apps/backend/src --ext ts --legacy-watch --poll-interval 1000 --exec "ts-node --project apps/backend/tsconfig.app.json -r tsconfig-paths/register apps/backend/src/main.ts"

[nodemon] 3.1.10
[nodemon] to restart at any time, enter `rs`
[nodemon] watching path(s): apps/backend/src/**/*
[nodemon] watching extensions: ts
[nodemon] starting `ts-node --project apps/backend/tsconfig.app.json -r tsconfig-paths/register apps/backend/src/main.ts --poll-interval 1000`
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [NestFactory] Starting Nest application...
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [InstanceLoader] TypeOrmModule dependencies initialized +69ms
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [InstanceLoader] PassportModule dependencies initialized +1ms
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [InstanceLoader] ConfigHostModule dependencies initialized +0ms
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [InstanceLoader] AppModule dependencies initialized +0ms
[Nest] 46  - 01/20/2026, 3:08:52 AM     LOG [InstanceLoader] ConfigModule dependencies initialized +1ms
[Nest] 46  - 01/20/2026, 3:08:52 AM   ERROR [ExceptionHandler] Neither apiKey nor config.authenticator provided
Error: Neither apiKey nor config.authenticator provided
    at Stripe._setAuthenticator (/app/node_modules/stripe/cjs/stripe.core.js:158:23)
    at new Stripe (/app/node_modules/stripe/cjs/stripe.core.js:96:14)
    at InstanceWrapper.useFactory [as metatype] (/app/apps/backend/src/payments/payments.module.ts:15:16)
    at Injector.instantiateClass (/app/node_modules/@nestjs/core/injector/injector.js:368:55)
    at callback (/app/node_modules/@nestjs/core/injector/injector.js:65:45)
    at async Injector.resolveConstructorParams (/app/node_modules/@nestjs/core/injector/injector.js:144:24)
    at async Injector.loadInstance (/app/node_modules/@nestjs/core/injector/injector.js:70:13)
    at async Injector.loadProvider (/app/node_modules/@nestjs/core/injector/injector.js:97:9)
    at async /app/node_modules/@nestjs/core/injector/instance-loader.js:56:13
    at async Promise.all (index 3)
[nodemon] app crashed - waiting for file changes before starting...

Changes Made

• Backend changes
• Frontend changes
• Database schema changes
• Configuration updates
• Other

Testing & Verification

• Unit tests pass
• Manual testing completed
• No breaking changes

Verification Steps:

Screenshots (if relevant)

Future Improvements/Notes

Related Issues

Closes #