lint-once: follow every spelling of docker build's file flag

The edge resolver matched `-f <file>` only. `docker build --file=X` fell
through to the default `Dockerfile` edge, so a second prettier pass wired in
that way was followed into the wrong file, counted nothing, and left the suite
green -- the exact false green this test exists to prevent, reachable by
writing the flag the long way.

Mutation, adding one line to the `check` recipe, before this commit:

    @docker build -f Dockerfile.lint .        1 failed / 47  caught
    @docker build --file=Dockerfile.lint .    48 passed / 48  MISSED

After, all four spellings fail with `expected 2 to be 1`:
`-f X`, `-f=X`, `--file X`, `--file=X`.

Docker takes the value either way for both the short and long flag, so the
resolver now reads `(?:-f|--file)[=\s]+`, still searched anywhere in the
invocation rather than at a fixed position. A leading \s keeps a longer flag
ending in the same letters (`--force-rm`) from supplying the match.

The header claimed `docker build -f <file>` coverage without qualification,
which a reader could take to include the long form it did not follow; it now
names all four forms and the fallback. The one limitation it asserts -- a
bundled cluster like `-qf X` resolving to the default -- is pinned by a test,
since an unpinned limitation is how the header drifts back into overclaiming.

test/packaging/lint-once.test.ts

@@ -18,8 +18,8 @@
 // `"$SCRIPT_DIR/<name>"` and `script/<name>` into other scripts, `make <target>`
 // and `$(MAKE) <target>` through the Makefile shims, the prerequisites of a
 // Makefile target, `yarn run <name>` through the `package.json` scripts, and
-// `docker build -f <file>` into that Dockerfile's `RUN` steps, heredoc bodies
-// included — and counts the prettier invocations it finds.
+// `docker build` into the `RUN` steps of the Dockerfile it names, heredoc
+// bodies included — and counts the prettier invocations it finds.
 //
 // What that covers is worth stating exactly rather than as "anywhere", because
 // a comment claiming more than the code delivers is the same defect as an
@@ -38,8 +38,13 @@
 // writes its recipes that way (`@$(YARN) tsc --watch`); the substitution is
 // one pass over literals, so a value that itself names another variable, a
 // value built by a make function (`$(shell ...)`, `$(addprefix ...)`), and the
-// body of a `define`/`endef` block are not expanded and not followed. Within
-// those edges, a prettier call is caught wherever it is added.
+// body of a `define`/`endef` block are not expanded and not followed. A
+// `docker build` resolves to the file named by `-f`, `-f=`, `--file` or
+// `--file=`, wherever in the invocation that flag sits, and to `Dockerfile`
+// when it names none; a Dockerfile chosen some other way — a bundled short
+// flag cluster (`-qf <file>`), a value this walk cannot expand to a literal —
+// resolves to the default rather than to the real file. Within those edges, a
+// prettier call is caught wherever it is added.
 //
 // Two entrypoints are walked, because they cover different graphs: `make check`
 // is what a developer runs, and `.gitea/workflows/check.yml` is what CI runs.
@@ -430,8 +435,18 @@ const edgesOf = (line: string): string[] => {
 
     // The container lint pass lives behind a `docker build`; without following
     // it the count would miss the one invocation that is supposed to survive.
+    //
+    // Every spelling of the file flag is read, not just `-f <file>`. Docker
+    // accepts `--file <file>`, `--file=<file>` and `-f=<file>` for the same
+    // thing, and matching `-f ` alone resolved all of them to the default
+    // `Dockerfile` edge — so `docker build --file=Dockerfile.lint .` in a
+    // recipe was followed into the wrong file, counted nothing, and reported
+    // green. The flag is searched for anywhere in the invocation, as `-f`
+    // already was, because a real build line wraps it in `--build-arg` and
+    // other flags. A leading `\s` is required so that a longer flag ending in
+    // the same letters cannot supply the match.
     if (/\bdocker\s+build\b/.test(line)) {
-        const file = /\s-f\s+(\S+)/.exec(line);
+        const file = /\s(?:-f|--file)[=\s]+(\S+)/.exec(line);
         edges.push(`docker:${file === null ? "Dockerfile" : file[1]}`);
     }
 
@@ -982,6 +997,61 @@ describe("the resolver reads what the shell would run", () => {
         );
     });
 
+    // `-f` is not the only spelling of the flag. `--file=Dockerfile.lint` and
+    // `--file Dockerfile.lint` mean exactly what `-f Dockerfile.lint` means,
+    // and matching only `-f` sent both to the default `Dockerfile` edge
+    // instead. That is the false-green shape this whole file exists to
+    // prevent: a second prettier pass added as
+    // `docker build --file=Dockerfile.lint .` was followed into the wrong
+    // Dockerfile, counted nothing, and left every build green. The long form
+    // is an ordinary thing for a human to write, so it is followed rather than
+    // merely documented as unfollowed.
+    //
+    // The equality rather than containment matters: resolving to the right
+    // file is only half of it, the default edge must not also be emitted.
+    it("follows --file in all three spellings as it does -f", () => {
+        expect(edgesOf("docker build --file=Dockerfile.lint .")).toEqual([
+            "docker:Dockerfile.lint",
+        ]);
+        expect(edgesOf("docker build --file Dockerfile.lint .")).toEqual([
+            "docker:Dockerfile.lint",
+        ]);
+        expect(edgesOf("docker build -f=Dockerfile.lint .")).toEqual([
+            "docker:Dockerfile.lint",
+        ]);
+    });
+
+    // The flag is found anywhere in the invocation rather than at a fixed
+    // position, matching how `-f` was already read, since a real build line
+    // carries `--build-arg` and friends around it.
+    it("finds the file flag after other flags and build args", () => {
+        expect(
+            edgesOf(
+                'docker build --build-arg LINT_EPOCH="1" --file=Dockerfile.lint --progress=plain .',
+            ),
+        ).toEqual(["docker:Dockerfile.lint"]);
+    });
+
+    // A different flag that merely begins with the same letters is not the
+    // file flag: `--force-rm` must not read as one, or its next token would be
+    // resolved as a Dockerfile and thrown on as unreadable.
+    it("does not mistake --force-rm for the file flag", () => {
+        expect(edgesOf("docker build --force-rm .")).toEqual([
+            "docker:Dockerfile",
+        ]);
+    });
+
+    // The header says a bundled short flag cluster resolves to the default
+    // rather than to the file it names, which is a limitation and so has to be
+    // pinned: an unpinned limitation is how the header starts overclaiming
+    // again. If someone teaches the resolver to split clusters, this test is
+    // the one that tells them to correct the header too.
+    it("resolves a bundled -qf cluster to the default Dockerfile", () => {
+        expect(edgesOf("docker build -qf Dockerfile.lint .")).toEqual([
+            "docker:Dockerfile",
+        ]);
+    });
+
     // This exact-equality assertion is also the block-scalar guard, which is
     // not obvious from its name: a `run: |` step resolves to the bare `|`,
     // which reaches nothing, so the prettier count would stay 1 no matter what