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

upaas uploads its clone as a tar context, which .dockerignore does not
filter, so its builds already carried .git; the binary said "unknown"
because the VERSION build arg defaulted to "unknown". The old .git/
exclusion kept .git out of a directory-context build only. .dockerignore
now lets .git through without its config, which can carry a credential,
and leaves out no tracked file (an excluded one would read as deleted and
mark the version -dirty). The VERSION build arg loses its "unknown"
default, so script/version derives the version inside the build; a given
VERSION still takes precedence.

The builder stage installs git, trusts the copied checkout whoever owns
its files, and fails when its context carries .git and the version still
comes out "unknown". The CI fingerprint is now the commit being checked.

Model: opus-5-5

internal/versionscript/version_script_test.go

@@ -115,8 +115,8 @@ func TestVersion_EnclosingRepositoryIsNotUsed(t *testing.T) {
 	require.Equal(t, unknown, runScript(t, inner, nil))
 }
 
-// The Docker build has no git metadata, so the version arrives as an
-// environment override. It wins over anything derivable.
+// An explicit VERSION, such as the Dockerfile's build arg, wins over
+// anything derivable.
 func TestVersion_EnvironmentOverrideWins(t *testing.T) {
 	t.Parallel()
 
@@ -128,8 +128,8 @@ func TestVersion_EnvironmentOverrideWins(t *testing.T) {
 }
 
 // An empty VERSION is treated as unset rather than stamping an empty
-// string: the Dockerfile's build arg has a non-empty default, but a
-// caller exporting VERSION= must not produce a binary reporting "".
+// string: a caller exporting VERSION= must not produce a binary
+// reporting "".
 func TestVersion_EmptyOverrideFallsBackToGit(t *testing.T) {
 	t.Parallel()
 
@@ -168,8 +168,8 @@ func TestMakefile_BuildComposesVersionAndExtraFlags(t *testing.T) {
 }
 
 // A caller can define VERSION as the empty string -- `make build
-// VERSION=`, or a `--build-arg VERSION=` reaching the Dockerfile's `make
-// build VERSION="$VERSION"`. script/version's own guard does not cover
+// VERSION=`, or the Dockerfile's `make build VERSION="$VERSION"` when no
+// VERSION build arg was given. script/version's own guard does not cover
 // that: the value never passes through the script. Stamping "" would
 // leave the binary reporting no version and the footer on "dev", which
 // is the defect this package exists for.
@@ -231,7 +231,7 @@ func TestDockerfile_BuildsThroughTheMakeTarget(t *testing.T) {
 
 	require.NotContains(t, dockerfile, "go build",
 		"a raw go build bypasses the Makefile's -X flag")
-	require.Contains(t, dockerfile, "ARG VERSION=")
+	require.Contains(t, dockerfile, "ARG VERSION")
 	require.Contains(t, dockerfile,
 		`make build VERSION="$VERSION" GO_LDFLAGS='-extldflags "-static"'`)
 }