Docker: fix building on Ubuntu 16.04 based images by forcing older PIP

Seems Python PIP v21 broke building on older base images that dont have
Python 3.6+.

Author: kmoppel

Dockerfile

@@ -2,7 +2,7 @@ FROM ubuntu:16.04
 
 RUN apt-get -q update \
     && apt-get -qy install wget apt-transport-https vim git postgresql postgresql-plpython3-9.5 postgresql-plpython-9.5 libfontconfig python3-pip python-pip libssl-dev libpq-dev \
-    && pip install -U pip && pip3 install -U pip \
+    && pip install -U "pip < 21.0" && pip3 install -U "pip < 21.0" \
     && locale-gen "en_US.UTF-8" && apt autoremove -y \
     && pg_dropcluster 9.5 main ; pg_createcluster --locale en_US.UTF-8 9.5 main \
     && echo "include = 'pgwatch_postgresql.conf'" >> /etc/postgresql/9.5/main/postgresql.conf

docker/Dockerfile-nonroot

@@ -2,7 +2,7 @@ FROM ubuntu:16.04
 
 RUN apt-get -q update \
     && apt-get -qy install wget apt-transport-https vim git postgresql postgresql-plpython3-9.5 postgresql-plpython-9.5 libfontconfig python3-pip libssl-dev libpq-dev \
-    && pip3 install -U pip
+    && pip3 install -U "pip < 21.0"
 
 RUN pip3 install supervisor \
     && locale-gen "en_US.UTF-8" && apt autoremove -y \