Force check layers to execute on every cibuild run (closes #89)

script/cibuild ran a bare "docker build .". The Dockerfile does "COPY . ."
and then runs the checks, so on an unchanged tree every check layer was a
cache hit: the suite never executed and the build still exited 0. A green
from script/cibuild did not mean the checks had passed, only that they had
passed at some point in the past.

Declare "ARG CHECK_EPOCH" in each stage that runs a check, positioned below
the dependency layers and immediately above the first check, and have
script/cibuild pass a fresh "$(date +%s)" on every invocation. A changed
build arg invalidates every layer below its declaration, so the checks
always execute while the base images, "go mod download" and the "yarn
install" in mdfmt stay cached.

All three check-running stages are covered: lint (fmt-check-go, lint),
mdfmt (prettier --check) and builder (test). ARG is scoped per stage, so a
stage without its own declaration would keep serving a cached pass and be
indistinguishable from a working fix at the exit code.

"--no-cache" was not used: it would also discard "go mod download" and the
yarn install, for no additional guarantee.

The README's build-status claim is accurate again and now says why.

Dockerfile

@@ -11,6 +11,14 @@ COPY . .
 # Touch .pb.go so make does not try to regenerate via protoc (file is committed)
 RUN touch mfer/mf.pb.go
 
+# Cache buster. script/cibuild passes a fresh CHECK_EPOCH on every invocation,
+# which invalidates every layer below this line so the checks always really
+# execute. Layers ABOVE it (base image, go mod download) keep their cache, so
+# this costs nothing but the checks themselves. Every stage that runs a check
+# needs its own copy: ARG is scoped per stage, and a stage without one silently
+# serves a cached pass. See https://git.eeqj.de/sneak/mfer/issues/89.
+ARG CHECK_EPOCH
+
 # Go half of fmt-check only: this image has no node, so no prettier. The
 # markdown half runs in the mdfmt stage below.
 RUN make fmt-check-go
@@ -27,6 +35,9 @@ RUN yarn install --frozen-lockfile
 
 COPY . .
 
+# Cache buster, so the check below always executes; see the lint stage.
+ARG CHECK_EPOCH
+
 # No make in this image; call the script entrypoint directly.
 RUN script/prettier --check
 
@@ -47,6 +58,9 @@ COPY . .
 # Touch .pb.go so make does not try to regenerate via protoc (file is committed)
 RUN touch mfer/mf.pb.go
 
+# Cache buster, so the check below always executes; see the lint stage.
+ARG CHECK_EPOCH
+
 RUN make test
 RUN cd cmd/mfer && go build -tags urfave_cli_no_docs -o /mfer .
 

README.md

@@ -23,8 +23,12 @@ javascript library is planned.
 
 # Build Status
 
-CI runs via `script/cibuild` (`docker build .`), which executes `make check`
-(formatting, linting, tests). The `main` branch must always be green.
+CI runs via `script/cibuild`, which builds the Dockerfile. Every stage that runs
+a check declares an `ARG CHECK_EPOCH` above it, and `script/cibuild` passes a
+fresh value on every invocation, so the check layers cannot be served from the
+Docker layer cache. A successful build therefore implies that formatting,
+linting, and tests actually ran and passed on the current tree. The `main`
+branch must always be green.
 
 # Entrypoints
 

script/cibuild

@@ -6,9 +6,14 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# A bare "docker build ." serves the check layers straight from the Docker
+# layer cache when the tree has not changed, so the build exits 0 without ever
+# running the checks. CHECK_EPOCH changes on every invocation and the
+# Dockerfile declares it above the checks in every stage that runs one, which
+# forces them to execute while leaving the dependency layers cached.
 main() {
     cd "$ROOT"
-    docker build .
+    docker build --build-arg CHECK_EPOCH="$(date +%s)" .
 }
 
 main "$@"