script/bootstrap installs the Dockerfile's Go from go.dev (closes #46)

The Gitea runner has Docker and git but no Go, and the apt path never
ran apt-get update, so on a fresh host every install failed.

Bootstrap now installs Go 1.26.8, the version in the Dockerfile's
golang image, from the release archive at go.dev, checked against a
sha256 in the script, into ~/.local/go whenever the go first on PATH
reports any other version. script/fmt, script/fmt-check,
script/precommit and the Makefile put ~/.local/go/bin first on their
PATH so they use it. The apt path runs apt-get update once, before its
first install.

Model: opus-5-5

Dockerfile

@@ -17,7 +17,8 @@ COPY . .
 RUN golangci-lint run --config .golangci.yml ./...
 
 # Test phase
-# golang:1.26-alpine, 2026-09-07
+# golang:1.26-alpine, 2026-09-07. It carries Go 1.26.8, the version
+# script/bootstrap installs on the host; change both together.
 FROM golang@sha256:ce864e7223ac17b1775e6fd0b4c0db580c2eb50e7953a427916379e4b92a1628 AS test
 
 # -race needs cgo, and cgo needs a C toolchain.