Use go modules

[sirianni]

Use go modules instead of legacy $GOPATH build.

Make some Dockerfile tweaks:

• Use ENTRYPOINT instead of CMD
• Add EXPOSE instruction

[Dabz]

Thanks a lot @sirianni for migrating to go module, that was something important to do! The image is building properly locally, so I will merge and this should kick-off a Dockerhub build.

It is worth noting that switching from CMD to ENTRYPOINT is actually a breaking change. Running ccloudexporter -cluster $CCLOUD_CLUSTER (previous way of running the ccloudexporter) would break the exporter :/

I guess it also highlights the fact that I do not have a proper versioning system in place yet :thinking: