Add scripts-to-rule-them-all scaffold (refs #1)

Bring the repo into conformance with the scripts-to-rule-them-all
(STRTA) scaffold. The real logic that lived inline in the Makefile now
lives in POSIX-sh entrypoints under script/, and the Makefile's standard
targets are thin @script/NAME shims.

- script/: bootstrap, setup, projectname, test, lint, fmt, fmt-check,
  check, docker, precommit, install-precommit, cibuild. All are
  executable #!/bin/sh entrypoints; the go mod tidy guard from the old
  inline hooks recipe moved into script/precommit.
- Makefile: the nine standard targets (bootstrap, setup, test, lint,
  fmt, fmt-check, check, docker, hooks) are now thin shims; the
  repo-specific sfdupes/build/clean targets and the CGO_ENABLED export
  are preserved.
- .gitea/workflows/check.yml: run script/cibuild instead of a bare
  docker build.
- Dockerfile: run make check (and the build) as an unprivileged builder
  user rather than root. We should never build or run as root, and doing
  so also lets the permission-denied tests run legitimately: root
  bypasses the chmod(0) that TestScanHardlinkRunFailsTogether relies on,
  which made the in-image make check fail. HOME and the Go caches point
  at the user's home so go build/test and golangci-lint can write.

make check passes locally and docker build . is green (the in-image
non-root make check passes, including the hardlink permission test).

script/docker

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/docker: build the Docker image tagged with the project name.
+# Identical in all repos; the tag comes from script/projectname.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build -t "$("$SCRIPT_DIR/projectname")" .
+}
+
+main "$@"