docs: name the two required flags, and record .dockerignore as part of the gate

Comments and documentation only; the docker build invocation and its flags
are untouched, and .dockerignore's effective rules are unchanged.

Two prose gaps from review. First, "both flags below must stay" had lost
its anchor: it ended a paragraph naming only --no-cache-filter, --target
was not introduced until the next one, and three flags follow on the
command, so a reader could pick the wrong pair. It now names --target and
--no-cache-filter explicitly.

Second, the list of things the tooling does not check covered the $stage
seam but not .dockerignore, which sits in the same trust boundary and is
the more likely thing to be edited — the first review on this change
actively suggested extending it for build artifacts. Only what reaches the
container is linted, so excluding a Go source there removes it from the
lint with no warning. Verified rather than asserted: a planted violation
plus that one path in .dockerignore yields `0 issues.` at exit 0 with the
violation still in the working tree, while excluding a file other code
still references fails loudly on `undefined:` typecheck errors instead.
The warning is recorded in script/lint alongside the $stage seam and in
.dockerignore itself, where the edit would actually be made.

script/lint

@@ -7,8 +7,9 @@
 #
 # --no-cache-filter forces the lint stage to re-execute every run, so an
 # unchanged tree is still actually linted; the deps stage keeps its cache,
-# so the module download is not repeated. Both flags below must stay: do
-# not "simplify" either one away.
+# so the module download is not repeated. It and --target must both stay,
+# for the reason in the next paragraph: do not "simplify" either of those
+# two away.
 #
 # The stage name is written ONCE, in $stage, and passed to both --target
 # and --no-cache-filter, so the two flags cannot come to name different
@@ -22,11 +23,24 @@
 # silent skip, because the one name reaches both flags.
 #
 # What the tooling does NOT check, and is left to whoever edits this:
-# $stage must name the stage in Dockerfile.lint that actually runs
-# golangci-lint. --target verifies that the name exists, not that it is
-# the right stage, and it stops the build at that stage — so moving the
-# lint step into a different stage, or adding a stage after this one,
-# would not be caught here. Keep this file and Dockerfile.lint in sync.
+#
+# 1. $stage must name the stage in Dockerfile.lint that actually runs
+#    golangci-lint. --target verifies that the name exists, not that it is
+#    the right stage, and it stops the build at that stage — so moving the
+#    lint step into a different stage, or adding a stage after this one,
+#    would not be caught here. Keep this file and Dockerfile.lint in sync.
+#
+# 2. .dockerignore decides what reaches the container, and only what
+#    reaches it gets linted. Excluding a Go file there removes it from the
+#    lint with no warning: verified by planting a real violation and adding
+#    just that file's path to .dockerignore, which produced `0 issues.` at
+#    exit 0 with the violation still sitting in the working tree. It shows
+#    up only if the rest of the package still references the excluded file,
+#    in which case the build fails loudly on `undefined:` typecheck errors;
+#    a self-contained file drops out silently. So .dockerignore is part of
+#    this gate, not housekeeping — keep it to build inputs the lint does
+#    not read, and never exclude Go sources, go.mod/go.sum or
+#    .golangci.yml.
 #
 # --output=type=cacheonly skips the image export. Nothing consumes the
 # image — the deliverable of this build is an exit code — and exporting it