fix: broken Dockerfile fix

[DanArmor]

Changed go image version according to updated dependencies in commit c8e729b6fed16f28aa69e01517eed68c92b1a5ca

Summary

Right now Dockerfile is broken, because c8e729b6fed16f28aa69e01517eed68c92b1a5ca bumped golang version to 1.21, but in Dockerfile 1.18 is used for build. go.mod file is using toolchain keyword, that was introduced in 1.21 - it leads into broken build:

Solution: updated version of the golang in Dockerfile to 1.21.8@sha256:856073656d1a517517792e6cdd2f7a5ef080d3ca2dff33e518c8412f140fdd2d

After that change build is working:

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