Keep modules, lint cache and system packages in check builds (closes #109)

Re-vendors REPO_POLICIES.md, script/cibuild, script/docker, script/lint
and script/test from sneak/prompts next at 0653579, on sneak's ruling in
sneak/prompts#130. In the lint, test and
builder stages of the Dockerfile, the module download, and in builder
the apk install, move above ARG CACHEBUST, so Docker keeps those layers
until the base image, the install line, go.mod or go.sum changes.
golangci-lint run also mounts golangci-lint's cache. Every step that
lints, tests or compiles still comes after ARG CACHEBUST, and go test
keeps -count=1 and -race. README and TODO.md say the same.

Model: opus-5-5

TODO.md

@@ -29,6 +29,15 @@
 
 # Completed Steps
 
+- re-vendor `REPO_POLICIES.md` and the four scripts that pass `CACHEBUST` from
+  `sneak/prompts` `next` at `0653579`: in the `lint`, `test` and `builder`
+  stages of the `Dockerfile` the module download, and in `builder` the
+  system-package install, now come before `ARG CACHEBUST`, so Docker keeps them
+  until the base image, the install line, `go.mod` or `go.sum` changes, and
+  every step that lints, tests or compiles still comes after it;
+  `golangci-lint run` also keeps golangci-lint's cache. README says the same
+  (2026-10-18, https://git.eeqj.de/sneak/sfdupes/issues/109)
+
 - releases until 1.0 are `v0.0.N` tags on `main`'s tip, so
   `go install sneak.berlin/go/sfdupes@latest` installs `main`'s tip once it has
   passed the build; README "Install" says so, says what `@main` and `v0.0.1`