Add workaround for Debian/Ubuntu venv setup failure

Signed-off-by: Joffrey F <joffrey@docker.com>

script/release/release.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-if test -d ./.release-venv; then
+if test -d ${VENV_DIR:-./.release-venv}; then
     true
 else
     ./script/release/setup-venv.sh
@@ -10,4 +10,4 @@ if test -z "$*"; then
     args="--help"
 fi
 
-./.release-venv/bin/python ./script/release/release.py "$@"
+${VENV_DIR:-./.release-venv}/bin/python ./script/release/release.py "$@"

script/release/setup-venv.sh

@@ -1,5 +1,11 @@
 #!/bin/bash
 
+debian_based() { test -f /etc/debian_version; }
+
+if test -z $VENV_DIR; then
+  VENV_DIR=./.release-venv
+fi
+
 if test -z $PYTHONBIN; then
   PYTHONBIN=$(which python3)
   if test -z $PYTHONBIN; then
@@ -16,15 +22,26 @@ if test $(echo $VERSION | cut -d. -f2) -lt 3; then
   echo "Python 3.3 or above is required"
 fi
 
-$PYTHONBIN -m venv ./.release-venv
+# Debian / Ubuntu workaround:
+# https://askubuntu.com/questions/879437/ensurepip-is-disabled-in-debian-ubuntu-for-the-system-python
+if debian_based; then
+  VENV_FLAGS="$VENV_FLAGS --without-pip"
+fi
 
-VENVBINS=./.release-venv/bin
+$PYTHONBIN -m venv $VENV_DIR $VENV_FLAGS
 
-$VENVBINS/pip install -U Jinja2==2.10 \
+VENV_PYTHONBIN=$VENV_DIR/bin/python
+
+if debian_based; then
+  curl https://bootstrap.pypa.io/get-pip.py -o $VENV_DIR/get-pip.py
+  $VENV_PYTHONBIN $VENV_DIR/get-pip.py
+fi
+
+$VENV_PYTHONBIN -m pip install -U Jinja2==2.10 \
     PyGithub==1.39 \
     pypandoc==1.4 \
     GitPython==2.1.9 \
     requests==2.18.4 \
     twine==1.11.0
 
-$VENVBINS/python setup.py develop
+$VENV_PYTHONBIN setup.py develop