Derive the image version from git; send .git to the build (closes #69)

The canonical documents told every repo to exclude .git from the build
context, default ARG VERSION to dev and never run git describe in a
build stage, so an image built from a clone with no build argument
reported dev. .dockerignore now sends .git. The Dockerfile example in
REPO_POLICIES.md installs git, takes the VERSION build argument when one
is given and otherwise git describe --tags --always, and fails when .git
is present but the version is empty, dev or unknown. The checklists, the
Go docs' Makefile comments, the README, and this repo's Dockerfile and
scripts now say the same.

Model: opus-5-5

script/cibuild

@@ -16,9 +16,8 @@ main() {
     "$SCRIPT_DIR/check"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
-    # excludes .git, so `git describe` in a build stage yields an empty
-    # version without failing.
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \