Derive the image's version from the .git in the build context (closes #366)

A plain docker build, as upaas runs it, passed no VERSION build arg and
had no .git, so every such image stamped "unknown". .dockerignore now
sends .git and every tracked file (an excluded one would read as
deleted and mark the version -dirty), and the VERSION build arg loses
its "unknown" default, so script/version derives the version inside the
build. A VERSION build arg still takes precedence.

The builder stage installs git and fails when its context carries .git
and the version still comes out "unknown".

Model: opus-5-5

Makefile

@@ -4,12 +4,12 @@
 .DEFAULT_GOAL := check
 
 # Version stamped into the binary. Derived from git by script/version;
-# override it (`make build VERSION=v1.2.3`) where git metadata is
-# unavailable, which is how the Dockerfile passes its build arg in.
+# override it (`make build VERSION=v1.2.3`) to stamp a given value, which is
+# how the Dockerfile passes its build arg in.
 VERSION ?= $(shell script/version)
 
-# An empty override (`make build VERSION=`, or a `--build-arg VERSION=`
-# landing on the Dockerfile's `make build VERSION="$VERSION"`) means unset,
+# An empty override (`make build VERSION=`, or the Dockerfile's `make build
+# VERSION="$VERSION"` when no VERSION build arg was given) means unset,
 # exactly as it does in script/version -- stamping "" would leave the binary
 # reporting no version and the footer back on its "dev" fallback. `override`
 # is required: a plain assignment loses to the command-line definition it