[IMP] use Python 3.7 in versin 12.0 to comply with OCA repos

odoo_openupgrade_wizard/templates/odoo/12.0/Dockerfile

@@ -1,9 +1,6 @@
-# <OOW> : Copy of https://github.com/odoo/odoo/blob/12.0/setup/package.dfsrc
-FROM debian:stretch
-
-RUN sed -i -- 's/security.debian.org/archive.debian.org/g' /etc/apt/**.list
-RUN sed -i -- 's/deb.debian.org/archive.debian.org/g' /etc/apt/**.list
-RUN sed -i -- 's/stretch-updates/stretch/g' /etc/apt/**.list
+# <OOW> : Copy of https://github.com/odoo/odoo/blob/12.0/setup/package.dfsrc,
+#         adapted to use Debian Buster (Python 3.7)
+FROM debian:buster
 
 RUN apt-get update && \
     apt-get install -y locales && \
@@ -22,7 +19,7 @@ RUN apt-get update -qq &&  \
         postgresql-server-dev-all \
         postgresql-client \
         adduser \
-        libsass0 \
+        libsass1 \
         libxml2-dev \
         libxslt1-dev \
         libldap2-dev \