docker: a plain docker build . stamps the git version (closes #210)

A plain `docker build .`, which is how upaas builds, stamped `dev`:
`.dockerignore` left out `.git` and the builder declared
`ARG VERSION=dev`. `.dockerignore` now sends `.git` without
`.git/config`, which can hold a credential, and lists no tracked file,
which git would count as deleted. `ARG VERSION` has no default. The
Makefile takes a non-empty `VERSION` from the command line or the
environment, so a build arg still wins; otherwise `git describe` runs in
the builder, which trusts the checkout whoever owns it, as a context
sent as a tar archive keeps its owners. A new `make version` prints the
version; the build fails when the context carries `.git` and it comes
out empty, `dev` or `unknown`.

Model: opus-5-5

Dockerfile

@@ -24,6 +24,11 @@ FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4
 
 RUN apk add --no-cache git make gcc musl-dev binutils-gold
 
+# A build context sent as a tar archive keeps its files' owners, and git
+# refuses to read a checkout owned by another user. Trust this one
+# whoever owns it.
+RUN git config --system --add safe.directory /src
+
 # Force BuildKit to run the lint stage before proceeding
 COPY --from=lint /src/go.sum /dev/null
 

README.md

@@ -592,15 +592,19 @@ appears in the startup log and in the health check response.
 
 The image takes it the same way, from the `.git` the build context carries, so a
 plain `docker build .` of a clone stamps the commit it was built from; a clone
-without tags stamps the short commit. A shallow clone carries only a tag on its
-own commit, so a shallow clone of an untagged commit stamps the short commit.
-`.dockerignore` sends `.git` without `.git/config`, which `git describe` does
-not need and which can hold a credential. A non-empty `--build-arg VERSION=...`
-takes precedence; `make docker` passes the version `git describe` gives on the
-host. The build fails when the context carries `.git`, as a directory or as a
-file, and the version comes out empty, `dev` or `unknown`. `.dockerignore` must
-list no tracked file: git in the build would see it as deleted and mark the
-version `-dirty`.
+without tags stamps the short commit. A clone made with `--depth 1` carries at
+most a tag on its own commit, so such a clone of an untagged commit stamps the
+short commit. In a build from a directory, `.dockerignore` keeps out
+`.git/config`, which `git describe` does not need and which can hold a
+credential. Docker does not apply `.dockerignore` to a context sent as a tar
+archive, as upaas sends it, so that context carries `.git/config` into the
+build. It also keeps its files' owners, so git in the build trusts the checkout
+whoever owns it. A non-empty `--build-arg VERSION=...` takes precedence;
+`make docker` passes the version `git describe` gives on the host. The build
+fails when the context carries `.git`, as a directory or as a file, and the
+version comes out empty, `dev` or `unknown`. `.dockerignore` must list no
+tracked file: git in the build would see it as deleted and mark the version
+`-dirty`.
 
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