Move from gableroux/unity3d to unityci/editor image

Breaking changes

• We're now using unityci/editor image instead of gableroux/unity3d in order to support better docker image versioning and introduce some fixes.
• This also means that licenses have to be reactivated when upgrading.
• Unity versions lower than 2018.2 are no longer supported (recommendation is to upgrade).

Changes

• Activation will now work across all unity versions within all editor images.
• Android builds have been confirmed to be successful again.
• Docker images are now versioned
• Automatically uses latest docker image version v0.x, at least while this is in alpha.

Pre 2018.2 support

• To use Unity versions before 2018.2 will require you to create a custom image. Instructions can be found on the repository for docker images (in the hub readme). Note that NDK and SDK will have to be manually configured.
• This approach is not actively supported by Unity CI, as our recommendation is to upgrade to at least an LTS version.

Credits

Thanks @BLaZeKiLL for putting in the work to convert the action to use the new image.