Enables cross platform building and pushing of docker images

[eelcocramer]

Would fix #628.

I've tried to keep as close as possible to the current workflow.

I added the make package step to have the builds for the various platform ready. I removed the line that deletes the build folders from the package script so that those artefacts can be reused by the docker build.

Finally building and pushing the docker containers is moved in a single step as the containers must be pushed directly by the buildx build.

I hope this will work in one go. I already tested the build but moved it out of the if condition in the script as I did not have the docker hub credentials setup.

[eelcocramer]

Some additional remarks:

• This workflow now builds packages for all platforms. This takes more time and not all platform packages are used for the docker build. However you may also use these packages for publishing the binaries in the future...
• There is a docker buildx action for GitHub workflows but I thought that it did not fit in with your current process.

[eelcocramer]

I've tested the build myself to my own docker hub account to verify if it works. There was an error which I fixed in a new commit. You can checkout the pipeline result in my fork and the result on docker hub.

Note that my pipeline has some additional changes to use my docker hub environment, these changes are not in this PR.

Also note that the version tag on the container in my docker hub account is just a bogus value I used for testing. I will remove the containers when this PR will be closed or merged.

[tidwall]

It all seems to work.
I'm gonna need to do some double checking to be sure, but so far it looks good.

[eelcocramer]

Thanks. Just let me know if you have any questions!