litestack:install should add database files to .dockerignore

[K4sku]

Rails 7.1 comes with a default Dockerfile and .dockerignore files.

Please correct me if I'm wrong, but it seems to me that running install generator should add the same exclusions to .dockerignore as to .gitignore. This way, users won't accidentally end up packing their development database into docker images.

Given the green light, I can cook up a PR for this.