Add support for s390x architecture - method 1

[sdarwin]

If you attempt to run a drone job on s390x architecture, it uses the docker image from https://hub.docker.com/r/drone/git which includes arm64 and windows however s390x is missing.

There are multiple solutions:
method 1. Leverage the docker image from thegeeklab
method 2. Run buildx on a vm
method 3. Rewrite everything to use buildx instead of manifest files. Won't explore this for now.

Probably method 1 is the simplest and easiest. That's in this PR. Method 2 is also viable and I will send it as a different pull request. Choose one or the other, not both.

I have tested both of these methods, and they work fine. Optionally, if you'd like to test the results, cloud.ibm.com offers instances at hourly rates so it's not difficult to launch an s390x virtual machine and try out the resulting docker image.

[sdarwin]

Hi,
The reason CI failed was not the code. Only permissions: "linter: untrusted repositories cannot enable privileged mode".

Drone is 99.9% of the way to supporting S390x already. Very little has to be done and it will automatically work. Two or three small changes are needed. This is one of those small changes. Would be great if it could be merged. Then we will look to any other issues. As a separate step.

You may be concerned regarding this pull request, that it should be necessary to do extensive end-to-end testing of S390x before proceeding. However, that's not the case. Consider that all this pull request is doing is adding one more architecture of drone-git on hub.docker.com. Focusing only on drone-git and docker, testing could be done as follows. Launch a small instance on cloud.ibm.com, pull the new docker image, and check if git runs properly. Not even that this should be automated, just manually once to observe it's working.