Add support for Python debugging in GDB

Switching from 3.6-stretch to 3.6.14-stretch. They are exactly the same image, but the latter is semantically better since the GDB Python script is for that patch version.

Dockerfile

@@ -9,7 +9,7 @@ WORKDIR /tcp_closer/src/build
 RUN cmake ..
 RUN make package
 
-FROM python:3.6-stretch
+FROM python:3.6.14-stretch
 RUN apt-get update && apt-get dist-upgrade -y && apt-get install -y \
       build-essential \
       libxml2-dev libxslt-dev zlib1g-dev libssl-dev libsqlite3-dev \
@@ -19,6 +19,10 @@ RUN apt-get update && apt-get dist-upgrade -y && apt-get install -y \
       build-essential autoconf automake libzmq3-dev libmnl0 make \
       && rm -rf /var/lib/apt/lists/*
 
+# GDB support for Python commands (e.g. py-bt)
+RUN mkdir -p /usr/share/gdb/auto-load/usr/local/bin/ && \
+    curl -L https://github.com/python/cpython/raw/v3.6.14/Tools/gdb/libpython.py -o /usr/share/gdb/auto-load/usr/local/bin/python3.6-gdb.py
+
 # Install tcp-closer
 COPY --from=builder /tcp_closer/src/build/tcp-closer-0.1.1-Linux.deb /tcp-closer.deb
 RUN dpkg -i /tcp-closer.deb