Docs: document build variables

docs/development/build-config.md

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+# Build Configuration
+
+When compiling Soapbox FE, environment variables may be passed to change the build itself.
+For example:
+
+```sh
+NODE_ENV="production" FE_BUILD_DIR="public" FE_BASE_PATH="/soapbox" yarn build
+```
+
+The following build variables are available:
+
+### `NODE_ENV`
+
+The environment to build Soapbox FE for.
+
+Options:
+- `"production"` - For live sites
+- `"development"` - For local development
+- `"test"` - Bootstraps test environment
+
+Default: `"development"`
+
+It's recommended to always build in `"production"` mode for live sites.
+
+### `FE_BUILD_DIR`
+
+The folder to put build files in. This is mostly useful for CI tasks like GitLab Pages.
+
+Options:
+- Any directory name, eg `"public"`
+
+Default: `"static"`
+
+### `FE_BASE_PATH`
+
+Subdirectory to host Soapbox FE out of.
+When hosting on a subdirectory, you must create a custom build for it.
+This option will set the imports in `index.html`, and the basename for routes in React.
+
+Options:
+- Any path, eg `"/soapbox"` or `"/fe/soapbox"`
+
+Default: `"/"`
+
+For example, if you want to host the build on `https://gleasonator.com/soapbox`, you can compile it like this:
+
+```sh
+NODE_ENV="production" FE_BASE_PATH="/soapbox" yarn build
+```