Introduce slim image for WP.org plugin deploys

debian/wp-org-deploy/Dockerfile

+FROM debian:stretch-slim
+
+LABEL maintainer="ethitter"
+LABEL version="1.0"
+
+RUN echo "deb http://security.debian.org/ stretch/updates main" >> /etc/apt/sources.list
+
+RUN apt-get update \
+    && apt-get -y --no-install-recommends install \
+        ca-certificates \
+        curl \
+        git \
+        rsync \
+        subversion \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*