Dockerfile: remove "goversion" stage

[AkihiroSuda]

No need to fetch https://golang.org/dl/$(cat /GOVERSION)... when we can just copy /usr/local/go from the golang:${GO_VERSION} image.

Alternative to:

• CI, build 4: Cleanup Dockerfile base images #3986

[apostasie]

This is increasing our reliance on Hub content instead of decreasing it, and adding unnecessary intermediary layers and dependent infrastructure with no added value.

[AkihiroSuda]

This is increasing our reliance on Hub content instead of decreasing it, and adding unnecessary intermediary layers and dependent infrastructure with no added value.

This PR just reuses an existing build-base-debian stage, so the reliance of Hub isn't changed, and it does not add a new intermediate layer.

[djdongjin]

LGTM

[apostasie]

This is increasing our reliance on Hub content instead of decreasing it, and adding unnecessary intermediary layers and dependent infrastructure with no added value.

This PR just reuses an existing build-base-debian stage, so the reliance of Hub isn't changed, and it does not add a new intermediate layer.

The point is:

• we use two distinct golang images from Hub
• we are now copying go from the debian image as-is to an ubuntu image - while presumably it is not a problem if all they are doing is retrieving it from golang server - and then if it is all they are doing, it then begs the question as to why we should rely on an extra intermediary piece of infrastructure (Hub) to do a curl for us (with availability delays and versioning scheme differences)

I guess we just disagree on this overall, so, this is fine, no point in beating this.

I just wish we would look at all of this critically.

We are currently retrieving about 6G of content from the network on a cold cache (out of which about 1G comes from Hub).
This is a very large amount of data that does not make much sense IMHO - and explains in part why we are getting 429 from Hub so regularly.

Anyhow.