CI: add semver tag for nightly docker image

[cyliu0]

Is your feature request related to a problem? Please describe.

We need semver tag to avoid unnecessary problems when testing nightly images with RisingWave Cloud. Refer to https://github.com/risingwavelabs/risingwave-cloud/pull/8833#issuecomment-2406591225

cc @arkbriar @Gogomoe @wjf3121 @lmatz @fuyufjh

Describe the solution you'd like

Add an additional developing semver tag based on the current branch and latest release like v2.1.0-alpha.20241011. We will keep the current nightly-YYYYMMDD tag as well.

Describe alternatives you've considered

No response

Additional context

No response

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