fix: copy plugins.json into prod image

[aldeed]

Resolves #6243
Impact: critical
Type: bugfix

Issue

The reactioncommerce/reaction:3.7.0 tag of the API Docker image won't start because one new file wasn't copied into it when it was built.

Solution

Update Dockerfile to copy plugins.json into the image.

Breaking changes

None

Testing

1. docker-compose down
2. Delete docker-compose.override.yml in this project if there is one.
3. Check out this branch
4. docker-compose up -d
5. docker-compose logs -f api and see the error from the bug report
6. docker-compose down
7. docker build -t reactioncommerce/reaction:3.7.0 . (there's a period at the end of this command)
8. docker-compose up -d
9. docker-compose logs -f api and verify the API started without error
10. docker-compose down