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 unknown default of the VERSION build arg is what stamped them. The image now derives its version itself: ARG VERSION has no default, so make build falls back to script/version, which runs git describe on the copied .git; a VERSION build arg still wins.

.dockerignore sends .git without its config in a directory context and no longer leaves out tracked files, which would mark the tree -dirty. The builder installs git, trusts /build as a safe.directory, and fails when .git is present but the version comes out unknown. A shallow single-branch clone stamps its short commit.

Model: opus-5-5

script/docker

@@ -2,9 +2,9 @@
 # script/docker: build the Docker image tagged with the project name.
 # The tag comes from script/projectname.
 #
-# .dockerignore excludes .git/, so the builder stage cannot derive the
-# version itself. It is resolved here, where the checkout is, and passed
-# in as a build arg; without it the image would stamp itself "unknown".
+# The version script/version resolves here goes in as the VERSION build
+# arg, which takes precedence over what the build would derive from the
+# .git in its context.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"