fix: broken Dockerfile fix

[DanArmor]

Changed go image version according to updated dependencies in commit c8e729b

Summary

Right now Dockerfile is broken, because c8e729b 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:

Ticket Link

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[mattermost-build]

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[DanArmor]

Encountered another error on startup, right after build

Changed go image version to golang:1.21.8-bullseye@sha256:81d98548f08e22a59c5c0be814acc0997f3dfd3313487adcf4e1d3d962af3a43

Now it works

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