x/build/devapp: handle module dependencies more intelligently in Dockerfile

[toothrot]

The x/build/devapp Dockerfile contains extra steps to optimize the speed of repeated docker builds when the go.mod or go.sum files have not changed. It would be nice to have a more automated way of doing this that doesn't require us to remember to add some our dependencies manually inside of our Dockerfile.

See golang.org/cl/193878.

[andybons]

What kind of speedup do we get from this optimization?

[dmitshur]

/cc @bradfitz Might be most familiar with the effect of this optimization.

[bradfitz]

The point is to speed up incremental development.

Checking now, it looks like it's ~11 seconds to do an increment "make docker-prod" after doing a minor change to x/build/devapp/*.go.

But without it, incremental builds are ~22 seconds, assuming you're on a really fast network (Google corp, wired). Because without those lines you pull lots of source from proxy.golang.org.

[bradfitz]

Btw, we used to have such a tool: gitlock. The current TODOs in our Dockerfiles come from removing that, in our move to using modules. (#26872)

See also: #27719

[gopherbot]

Change https://golang.org/cl/197302 mentions this issue: cmd/tip: don't install cmd/tip from the internet