RAS-1976: Fix cross-building Docker images

[prem-buczkowski-ons]

What and why?

The final stage was pinned to --platform=$BUILDPLATFORM and go build never set GOARCH, so build of the arm64 image in CI/CD built for amd64 and labelled it arm64 nevertheless - and it failed to run with an "exec format error". Add ARG TARGETARCH + GOOS/GOARCH, unpin the final stage.

(If you built on a Mac, then you'd get arm64 for both instead, so the code seemed to work.)

Nicely, it means that the build stage will cross-compile on the native compiler without emulation, so the build stage for both arm64 and amd64 should take roughly the same time, no matter the architecture of the build machine.

Also swap the runtime image to alpine:3.24 (~250MB -> ~10MB), adding the ca-certificates and CGO_ENABLED=0 that were implicit in golang:*-alpine.

How to test?

# architecture check of the main binary in the image
docker buildx build --platform linux/arm64 --output type=tar,dest=- \
  -f _infra/docker/Dockerfile . \
  | tar -xOf - opt/survey/main \
  | { dd bs=1 skip=18 count=2 2>/dev/null; cat >/dev/null; } \
  | od -An -tx1

# the same, but for amd64
docker buildx build --platform linux/amd64 --output type=tar,dest=- \
  -f _infra/docker/Dockerfile . \
  | tar -xOf - opt/survey/main \
  | { dd bs=1 skip=18 count=2 2>/dev/null; cat >/dev/null; } \
  | od -An -tx1

It should say b7 00 for arm64, and 3e 00 for x86-64 (as it did for both before).

Jira

https://officefornationalstatistics.atlassian.net/browse/RAS-1976

[arroyoAle]

/gcbrun