refactor: use multi-stage Dockerfile for smaller runtime image

Dockerfile

@@ -1,13 +1,21 @@
-FROM python:3.12-slim
+FROM python:3.12-slim AS builder
 
 RUN apt-get update && \
-    apt-get install -y postgresql-client gcc libpq-dev gzip && \
+    apt-get install -y gcc libpq-dev && \
     rm -rf /var/lib/apt/lists/*
 
 RUN pip install --upgrade pip setuptools wheel
 
 COPY requirements.txt /app/requirements.txt
-RUN pip install --no-cache-dir -r /app/requirements.txt
+RUN pip install --prefix=/install -r /app/requirements.txt
+
+FROM python:3.12-slim
+
+RUN apt-get update && \
+    apt-get install -y postgresql-client gzip && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /install /usr/local
 
 COPY main.py /app/main.py