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

Docker does not apply an app's `.dockerignore` to a build context sent as a tar, which is how upaas sends it, so every file in the clone, `.git/config` included, reached the build.

upaas now reads the ignore file as `docker build` does, with the `ignorefile` reader of `github.com/moby/patternmatcher`, and leaves those files out of the tar: an ignore file named after the Dockerfile and next to it, such as `Dockerfile.dockerignore`, otherwise `.dockerignore` at the root of the clone. The Dockerfile path is read as a path inside the clone, and the Dockerfile (or the lowercase `dockerfile` Docker builds when `Dockerfile` is missing) and the ignore file always stay in. An app without an ignore file builds as before.

Not handled: a Dockerfile that is a symlink to an ignored file fails the build.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -268,6 +268,12 @@ upaas fails the deploy instead of building. A Dockerfile that uses
 `RUN --network` needs Docker Engine 23.0 or later unless its `# syntax=` line
 names Dockerfile frontend 1.3 or later, such as `docker/dockerfile:1`.
 
+The build context leaves out the files the app's ignore file names, as
+`docker build` does: `<Dockerfile>.dockerignore` next to the app's Dockerfile if
+there is one, otherwise `.dockerignore` at the root of the repository. The
+Dockerfile and `.dockerignore` are always sent, even when the ignore file names
+them.
+
 Session secrets are automatically generated on first startup and persisted to
 `$UPAAS_DATA_DIR/session.key`.