[docker] Update root Dockerfile (#6870)

Dockerfile

@@ -1,22 +1,31 @@
 FROM jimschubert/8-jdk-alpine-mvn:1.0
 
-ENV GEN_DIR /opt/swagger-codegen
-
 RUN set -x && \
     apk add --no-cache bash
 
-RUN mkdir /opt
+ENV GEN_DIR /opt/swagger-codegen
+WORKDIR ${GEN_DIR}
+VOLUME  ${MAVEN_HOME}/.m2/repository
 
-ADD . ${GEN_DIR}
+# Required from a licensing standpoint
+COPY ./LICENSE ${GEN_DIR}
 
-VOLUME  ${MAVEN_HOME}/.m2/repository
+# Required to compile swagger-codegen
+COPY ./google_checkstyle.xml ${GEN_DIR}
 
-WORKDIR ${GEN_DIR}
+# Modules are copied individually here to allow for caching of docker layers between major.minor versions
+# NOTE: swagger-generator is not included here, it is available as swaggerapi/swagger-generator
+COPY ./modules/swagger-codegen-maven-plugin ${GEN_DIR}/modules/swagger-codegen-maven-plugin
+COPY ./modules/swagger-codegen-cli ${GEN_DIR}/modules/swagger-codegen-cli
+COPY ./modules/swagger-codegen ${GEN_DIR}/modules/swagger-codegen
+COPY ./pom.xml ${GEN_DIR}
 
+# Pre-compile swagger-codegen-cli
 RUN mvn -am -pl "modules/swagger-codegen-cli" package
 
+# This exists at the end of the file to benefit from cached layers when modifying docker-entrypoint.sh.
 COPY docker-entrypoint.sh /usr/local/bin/
 
 ENTRYPOINT ["docker-entrypoint.sh"]
 
-CMD ["build"]
+CMD ["help"]