MAINT: update docker image

.docker-base/Dockerfile

 # Use docker as parent image
-FROM docker:19.03.0
+FROM docker:20.10
 
 # http://bugs.python.org/issue19846
 ENV LANG C.UTF-8
 
 # install dependencies
 RUN apk add --no-cache py3-pip python3 python3-dev gcc make \
-    git bash curl gettext  py3-requests 
+    git bash curl gettext  py3-requests
 RUN apk add --no-cache libffi-dev openssl-dev libc-dev libxslt libxslt-dev \
     libxml2 libxml2-dev