Require docker image for ARM64

Created by: odidev

Hi Team,

I am checking on adding arm64 support in Travis-CI for the Open Service Broker API package. But I was facing an issue in pulling openapitools/openapi-generator-cli image for arm64 as it is not available in the docker hub. So I tried to build an openapi-generator image locally on arm64 machine and found that 3.6.3-jdk-11-openj9 image is not available for arm64. So I replaced this image with 3.6.3-jdk-11 and I was successful in building an openapi-generator docker image in my local arm64 machine.

If you are interested in uploading an arm64 image, I can raise a PR for the changes required to build an arm64 image. It will be very helpful if this image is available for the arm64 platform.