fix: include README.md in Docker build context

hatchling requires README.md at build time but it was
not being copied into the builder stage.

Author: Patel230

Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3.12-slim AS builder
 
 WORKDIR /build
-COPY pyproject.toml VERSION ./
+COPY pyproject.toml VERSION README.md ./
 RUN pip install --no-cache-dir build
 
 COPY src/ src/