Add hint that docker repository names (git org + git repo name) must be all lowercase

[ianfixes]

The documentation fails to mention that OWNER or REPOSITORY substitution with a name that contains a capital letter will fail.

Logging in to registry docker.pkg.github.com
WARNING! Using -*** the CLI is insecure. Use --password-stdin.
WARNING! Your password will be stored unencrypted in /github/home/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

Login Succeeded
Building image [docker.pkg.github.com/Arduino-CI/action/:latest]
invalid argument "docker.pkg.github.com/Arduino-CI/action/:latest" for "-t, --tag" flag: invalid reference format: repository name must be lowercase
See 'docker build --help'.
Error: exit status 125

invalid reference format: repository name must be lowercase

see https://github.com/Arduino-CI/action/runs/1508611501#step:4:23

[welcome]

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[ianfixes]

As I continue to work on this, I notice docker/build-push-action#127 (comment)

docker.pkg.github.com is deprecated and will sunset early next year

Also, docker/build-push-action@v2 is wildly different from @v1.

[janiceilene]

@ianfixes Thanks for opening a PR 👍 Please fill out the pull request template in the top comment in this pull request to help us better understand the context of your changes, then we'll get this up for review!

[ianfixes]

I'm not sure what the issue would say except that the docs for uploading docker images are out of date and incomplete. As per my previous comment, I'm not even sure what the scope of the fix should be