Stamp the tag or short commit in a plain docker build (closes #112)

.dockerignore now sends .git but not .git/config, which can hold a
credential and which git describe does not need. The build stage stamps
main.Gitrev from the VERSION build argument when one is given, otherwise
from git describe --tags --always, and fails if .git is present and no
version comes out. git there trusts /src whoever owns it, since a
context sent as a tar archive keeps its files' owners. script/docker is
replaced by the canonical copy, which passes VERSION; bin/gitrev.sh uses
--tags too, so every entrypoint stamps the same value for a clean
commit.

Model: opus-5-5

script/docker

@@ -1,7 +1,8 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
-# Generic: needs no adaptation.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -9,7 +10,15 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"