Let's add a simple guide for a custom image text written by user.

[lizable]

⚠️ FYI: This feature is suggested by @adrysn.

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] 기능 요청이 특정 문제에 연관된 것이라면 여기에 그 문제를 설명해주세요.

Since, the feature about creating a session with a custom image (manual image) is implemented via #1053, It would be great to provide a format guide of the custom image in the session launcher so that users can create sessions using a manual image more easily.

Describe the solution you'd like A clear and concise description of what you want to happen. 어떤 기능이 있으면 좋겠는지 자세히 설명해주세요. There are some items that need to be in the guide.

• A number cannot appear first after a hyphen(-) in an image tag.

And also there are two options to show this guide.

• Option 1: check whether the input value follows the guide and if not, treat it as an invalid value.
• Option 2: Provide a guide that can be shown by clicking an icon (such as an information icon) or using a tooltip.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered. 혹시 다른 대안들을 생각해본 적이 있다면 함께 적어주세요.

Additional context Add any other context or screenshots about the feature request here. 기능 요청에 대해 보다 잘 이해할 수 있는 다른 맥락을 기술해주세요.

• Valid image name: cr.backend.ai/stable/python:3.8-ubuntu18.04
• Invalid image name: cr.backend.ai/stable/pytonh:3.8-18.04 <- number starts right after -(hyphen)