fix(docker): multi-stage build and publish tags

- Split Dockerfile into builder and runtime stages
- Put .venv on PATH so pymodbus resolves in the container
- Push image to Docker Hub and GHCR with latest and 7-char SHA tags
- Use real repo/image names in README

Dockerfile

@@ -1,10 +1,18 @@
-FROM python:3.11-slim
+# Stage 1: build and install dependencies
+FROM python:3.11-slim AS builder
 
 WORKDIR /app
 
-# Install dependencies
 COPY pyproject.toml uv.lock ./
-RUN pip install uv && uv sync --frozen
+RUN pip install --no-cache-dir uv && uv sync --frozen --no-dev --no-install-project
+
+# Stage 2: runtime (slim, no build toolchain)
+FROM python:3.11-slim AS runtime
+
+WORKDIR /app
+
+COPY --from=builder /app/.venv ./.venv
+ENV PATH="/app/.venv/bin:$PATH"
 
 # Copy application
 COPY main.py ./