docs: Update Docker image version from 'latest' to 'main'

[kyungilcho]

Update the Docker image version in the documentation to reflect the actual available version. The 'latest' tag was not available, causing confusion for users. The 'main' tag should be used instead.

[youngjoon-lee]

Let's keep using latest. The main tag shouldn't be used by external users because it's only for internal CIs. Instead, we need to create a proper git tag from our latest stable commit and publish a docker image with latest tag.

[gyuguen]

I think our documentation should use the latest docker image version. I'll close this PR.