Docker workflow use separate username for login

[FintasticMan]

@JF002 This pull request fixes the issue mentioned here about that the username used for logging in to Docker Hub is different than the one that actually has the images. This works around it by having 2 secrets: DOCKER_HUB_LOGIN_USERNAME, which should be set to (in this case) jf002, and DOCKER_HUB_IMAGE_USERNAME, which should be set to infinitime.

I don't have an organisation on Docker Hub to test it out on, but I have tested it on my personal repository, with both secrets set to fintasticman, and it works fine.

[JF002]

@FintasticMan So, yesterday, I modified the workflow file so I can run the workflow manually.
Then, I ran it manually. It did the build but did not publish the image on docker hub (https://github.com/InfiniTimeOrg/InfiniTime/runs/7263114629?check_suite_focus=true). I guess I need to add a condition in those if statement. I'll try to have a look at it asap :-)

[FintasticMan]

I think it would be best to change it from github.event_name == 'push' to github.event_name != 'pull_request'. This would mean that it would push to Docker Hub on everything other than pull_request, which is just pushes and manual triggers.

I'll make a pull request that does that.