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

> ⚠️ 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)`

JIRA Issue: FR-164