Let the image build see the dependency Cargo patches

The rename pass vendored a patched sieve-rs and pointed Cargo.toml's
[patch.crates-io] at vendor/sieve-rs. .dockerignore ignores everything and
re-includes a short list that did not have vendor on it, so the image build
had no such directory and stopped at

    failed to load source for dependency `sieve-rs`
    failed to read /build/vendor/sieve-rs/Cargo.toml

CI could not have caught that: it builds from a checkout, where the
directory is simply there, and only the image build has a context to prune.
The first that was known about it was a tag that had already been pushed.

So: vendor is re-included, and tools/fork/context-check.py now asserts the
thing that was quietly assumed -- every path a [patch] section names exists
and survives .dockerignore. It runs beside the other fork checks and takes
no toolchain.

Also, the comments in .dockerignore started with // , which Docker does not
read as a comment: they were patterns that happened to match nothing. They
are # now.

.dockerignore

@@ -1,10 +1,16 @@
-// Ignore everything
+# Ignore everything
 *
 
-// Allow what is needed
+# Allow what is needed
 !crates
 !tests
 !resources
 
+# The patched dependency Cargo.toml's [patch.crates-io] points at. Without
+# it the build context has no vendor/, and `cargo chef cook` fails on
+# "failed to load source for dependency sieve-rs" -- which CI cannot see,
+# because CI builds from a checkout and only the image build has a context.
+!vendor
+
 !Cargo.lock
 !Cargo.toml