Add Bitbucket Server to GitHub Enterprise migration tool

.gitignore

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+__pycache__/
+*.pyc
+*.egg-info/
+dist/
+build/
+.pytest_cache/
+config.yaml
+state.json
+*.egg

Dockerfile

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+FROM python:3.12-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    git \
+    openssh-client \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY . .
+RUN pip install --no-cache-dir -e .
+
+# SSH config for non-interactive git clone
+RUN mkdir -p /root/.ssh && \
+    echo "StrictHostKeyChecking no" >> /root/.ssh/config
+
+ENTRYPOINT ["bb2gh"]
+CMD ["--help"]