Stamp the tag or short commit in a plain docker build (closes #166)

.dockerignore now lets .git into the build context, without
.git/config, which can hold a remote URL with a credential. ARG VERSION
has no default: given none, the build stage takes the version from
git describe --tags --always, and fails if the context carries .git
and no version comes out. pixad now logs its name, version and
architecture as its first log line, through the existing
Logger.Identify, which nothing called.

Model: opus-5-5

.dockerignore

@@ -1,4 +1,7 @@
-# .git is sent so the build can stamp the version, without its config.
+# .git is sent without its config. Without a VERSION build argument the
+# stage that compiles runs `git describe --tags --always` on .git, which
+# does not need .git/config; that file can hold a credential, such as a
+# password in a remote URL or the token the CI checkout step stores there.
 .git/config
 .gitignore
 .DS_Store