Leave out of the build context what the app's .dockerignore names (closes #274)

Docker does not apply .dockerignore to a build context sent as a tar,
so upaas sent every file in the clone. The build now reads the ignore
file as docker build does, <Dockerfile>.dockerignore next to the
Dockerfile if there is one, otherwise .dockerignore at the root, with
the ignorefile reader of moby/patternmatcher, and passes its patterns
as exclude patterns. As the docker command line does, the Dockerfile
and .dockerignore are never left out. The ignore file is read through
os.Root, so it cannot be read from outside the clone. An ignore file
that cannot be read, or a malformed pattern, fails the build.

Model: opus-5-5

TODO.md

@@ -20,6 +20,13 @@ regress.
 
 # Completed Steps
 
+- 2026-10-03: An app's build context leaves out the files its `.dockerignore`
+  names, such as `.git/config`, as `docker build` does; before, every file in
+  the clone was sent. An ignore file next to the Dockerfile,
+  `<Dockerfile>.dockerignore`, is read instead when there is one. The Dockerfile
+  and `.dockerignore` are always sent. An ignore file that cannot be read, or
+  holds a pattern Docker rejects, fails the build (#274).
+
 - 2026-10-02: In a window too narrow for the top bar, such as 390 px, the New
   App and Logout buttons move to a second row instead of running into "by
   @sneak"; the bar keeps a gap between its two sides at every width (#272).