add a heroku-18 Dockerfile

builds/Dockerfile.heroku-18

+FROM heroku/heroku:18-build
+
+WORKDIR /app
+ENV WORKSPACE_DIR="/app/builds" \
+    S3_BUCKET="lang-python" \
+    S3_PREFIX="heroku-18/"
+
+RUN apt-get update && apt-get install -y python-pip && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt /app/
+RUN pip install --disable-pip-version-check --no-cache-dir -r /app/requirements.txt
+
+COPY . /app/builds/