Fix build of 1.2 on Bionic (ref spotify#306)

* Adds an "opts" build option to allow more compatible builds
  (no Sphinx dependencies)
* Installs debhelper 12 on Bionic from backports
* Fixes minor details that allow using the Dockerfile on Windows,
  because of...

  You are building a Docker image from Windows against a non-Windows
  Docker host. All files and directories added to build context will
  have '-rwxr-xr-x' permissions. It is recommended to double check
  and reset permissions for sensitive files and directories.

  Since this is a pure builder image, that is not an actual problem.

Dockerfile

@@ -1,21 +1,31 @@
 # Build dh-virtualenv's Debian package within a container for any platform
 #
-#   docker build --tag dh-venv-builder --build-arg distro=debian:9 .
+#   docker build --tag dh-venv-builder --build-arg distro=debian:10 .
 #   docker build --tag dh-venv-builder --build-arg distro=ubuntu:bionic .
 #
-#   mkdir -p dist && docker run --rm dh-venv-builder tar -C /dpkg -c . | tar -C dist -xv
+#   mkdir -p dist && command docker run --rm dh-venv-builder tar -C ../dpkg -c . | tar -C dist -xv
+#
+# Add '--build-arg opts=nodoc' to remove dependencies on Sphinx packages not available in older releases.
 
 ARG distro="debian:stable"
+ARG opts=""
+
 FROM ${distro} AS dpkg-build
+ENV DEB_BUILD_OPTIONS=${opts}
 RUN apt-get update -qq -o Acquire::Languages=none \
     && env DEBIAN_FRONTEND=noninteractive apt-get install \
         -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
         build-essential debhelper devscripts equivs lsb-release libparse-debianchangelog-perl \
         python3 python3-setuptools python3-pip python3-dev \
         python3-sphinx python3-mock dh-exec dh-python python3-sphinx-rtd-theme \
+    && if test "$(lsb_release -cs)" = 'bionic' ; then \
+        apt-get install -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
+                        -t bionic-backports debhelper; fi \
     && apt-get clean && rm -rf "/var/lib/apt/lists"/*
 WORKDIR /dpkg-build
 COPY ./ ./
+# The "chmod" call fixes '-rwxr-xr-x' permission problems you get when running this builder from Windows.
 RUN sed -i -re "1s/..unstable/~$(lsb_release -cs)) $(lsb_release -cs)/" debian/changelog \
+    && chmod a-x debian/dh-virtualenv.* \
     && dpkg-buildpackage -us -uc -b && mkdir -p /dpkg && cp -pl /dh-virtualenv[-_]* /dpkg \
     && dpkg-deb -I /dpkg/dh-virtualenv_*.deb