go: docker: change entrypoint name

[kamilsk]

Motivation: make it more consistent with toolchain.

E.g.,

module go.octolab.org/module

go 1.20

toolchain go1.20.10

golang has that tag https://hub.docker.com/_/golang/tags

So, I can use the same directive toolchain go1.20.10 -> make go1.20.10, and automate the call: make toolchain (it reads toolchain and use appropriate image).

Problem: there is no 1.20.0 image tag.