Update validator.Dockerfile to match converted.Dockerfile updates

Author: jakebeal

docker/validator.Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-alpine
+FROM python:3.9-alpine
 
 RUN apk add nginx openjdk8 python3-dev build-base linux-headers pcre-dev bash
 
@@ -10,6 +10,7 @@ COPY docker/nginx-validator.conf /etc/nginx/nginx.conf
 COPY docker/sbol-validator.ini /opt/SBOL-Validator/src/sbol-validator.ini
 COPY docker/run.sh /opt/SBOL-Validator
 
+RUN pip install --upgrade pip
 RUN pip install -r requirements.txt
 RUN chmod -R 777 .