Add Dockerfile with multi-stage build

[nileger]

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What

I found it extraordinarily difficult to build and run the mq_prometheus exporter using the provided Dockerfiles and shell scripts. The environment variables, paths, and copying files from and to the host make the entire process cumbersome.

Hence, I added a Dockerfile that enhances the build process. It uses multi-stage builds, which is the state-of-the-art and preferred way, compared to using multiple Dockerfiles and copying files from Docker container to host and vice versa.

Now, we must simply run one single command. All steps are executed irrespective of the underlying host. Hence, we do not need different scripts for Linux and Windows machines.

How

The Dockerfile consists of two stages. The first stage builds the Go application and copies the binary to the second stage, which builds a runtime for the Go application.

Testing

Run docker build -t {tag}:{version} . in the root folder of the repository. Then, start the Docker image, e.g., with ./scripts/docker-compose.yml.

Issues

This pull-request is not linked to any currently open issues.

[nileger]

@ibmmqmet
I haven't updated the CHANGELOG and the README yet because I'm not sure whether the two existing Dockerfiles and some of the shell scripts are required any longer. What do you think?

[ibmmqmet]

Thanks for this.

I'll probably leave the existing scripts and Dockerfiles alone for a while because they do demonstrate some varied ways in which you might want to actually run the programs - not everyone wants to execute in a container, though they might want to build that way. For example, if you want to run the collectors as MQ SERVICE programs locally you will need the actual binaries (and associated resources like config files) outside the container.

I'll update the README/CHANGELOG separately as part of the next update.