fix: docker/build/Dockerfile.source to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DEBIAN9-CURL-466505
- https://snyk.io/vuln/SNYK-DEBIAN9-CURL-466508
- https://snyk.io/vuln/SNYK-DEBIAN9-CURL-466508
- https://snyk.io/vuln/SNYK-DEBIAN9-DPKG-2847943
- https://snyk.io/vuln/SNYK-DEBIAN9-FREETYPE-1019584

docker/build/Dockerfile.source

@@ -10,7 +10,7 @@ WORKDIR /deploy/sakai
 RUN git checkout ${release} && mvn clean install sakai:deploy -Dmaven.test.skip=true
 
 
-FROM tomcat:9.0.20-jre8
+FROM tomcat:9.0.86-jre8
 
 COPY lib/server.xml /usr/local/tomcat/conf/server.xml
 COPY lib/context.xml /usr/local/tomcat/conf/context.xml