fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2004-EXPAT-2403778
- https://snyk.io/vuln/SNYK-UBUNTU2004-EXPAT-2403934
- https://snyk.io/vuln/SNYK-UBUNTU2004-NGHTTP2-5954795
- https://snyk.io/vuln/SNYK-UBUNTU2004-OPENSSL-1569470
- https://snyk.io/vuln/SNYK-UBUNTU2004-OPENSSL-2426343

Dockerfile

@@ -1,4 +1,4 @@
-FROM maven:3.6.3-jdk-11-openj9
+FROM maven:3.8-jdk-11-openj9
 
 ENV GEN_DIR /opt/openapi-generator
 WORKDIR ${GEN_DIR}