Keep secrets out of the Docker build context at every depth (closes #29)

The canonical .dockerignore was three lines -- .git, node_modules, .DS_Store
-- while the canonical Dockerfile does `COPY . .`, so a developer's local
.env, *.pem or *.key was shipped into the build context and could land in an
image layer. Nothing surfaced it because .gitignore covers those patterns, so
the files are invisible to every git-based check.

The obvious repair, copying .gitignore's secret patterns across, is worse than
the gap it closes. .dockerignore does not use .gitignore semantics: Docker
matches with Go filepath.Match, `*` does not cross `/`, and a pattern without
a leading `**/` is anchored at the build-context root. A file listing .env,
*.pem and *.key therefore reads as solved, reviews as solved, and protects
only the repository root, while config/.env and certs/server.key still ship.
The three-line file at least invited scrutiny; the transplanted form
manufactures confidence and stops anyone looking.

So every depth-independent pattern here carries the `**/` prefix and only
genuinely root-anchored entries stay unprefixed. `**/node_modules` fixes a
defect the three-line file had today for any nested node_modules,
independently of the secret exposure.

The OS and editor patterns are included on their own merits rather than by
mirroring .gitignore. None of them is ever a build input, and editor state in
particular churns under a developer's hands, so each one is a source of
`COPY . .` invalidation carrying no information about the source tree. Now
that the checks are keyed on CHECK_EPOCH rather than on accidental context
churn, there is no reason left to keep churn in the context. Language build
artifacts are deliberately absent: they are per-repo, and the file's header
comment tells consuming repos to add their own host-built binaries, which is
the case that actually bites -- a host `make build` drops a multi-megabyte
artifact into the context where .gitignore hides it from every git-based
check.

.gitignore is untouched. Its semantics are the inverse: an unanchored pattern
already matches at any depth, so `**/`-prefixing it produces a file that is
wrong in a way that looks careful. That asymmetry is why "derive one from the
other" was the wrong instruction, and it is now written down in
REPO_POLICIES.md in both directions, together with the requirement to verify
by enumerating the image rather than by reading the patterns. Every consuming
repo inherits .dockerignore by copy, so the trap has to live where the next
person looks, not only be fixed once here. Both repo checklists gain the same
requirement, since they are what an agent reads while extending the file.

Verified by planting .env, server.key and ca.pem at the root plus config/.env,
config/.env.production, certs/ca.pem, certs/server.key,
deploy/secrets/id_rsa.key, web/node_modules/nested/index.js and a nested .swp
below it, then building a standalone probe image doing `COPY . .` and listing
what actually landed inside it. Before: all eleven planted files in the image.
Against the naive unprefixed form: the three root-level files excluded and
every nested one still present, which is what shows the enumeration can detect
the failure mode at all. After: every planted file excluded at every depth,
with web/src/app.js still present to prove the probe was copying nested files
rather than copying nothing.

Transferred-context size is recorded but load-bearing on nothing, and the runs
show why: the naive build reported 2.18kB transferred while 43 files, five of
them secrets, were in the image. BuildKit transfers only the delta from the
previous build, so the number describes the transfer and not the contents.

Planted files were removed and their absence confirmed against the filesystem
rather than against `git status`, which could not have seen them.

`make docker` re-run after the change: the check layer executed rather than
being served from cache, so the CHECK_EPOCH verification still holds under the
altered build context.

.dockerignore

@@ -1,3 +1,36 @@
+# .dockerignore uses Go filepath.Match, NOT .gitignore semantics: `*`
+# does not cross `/`, and a pattern without a leading `**/` is anchored
+# at the build-context root. Every depth-independent pattern therefore
+# needs the `**/` prefix — without it `config/.env` and
+# `certs/server.key` still ship while the file reads as solved. Entries
+# that are genuinely root-anchored stay unprefixed. Extend this file
+# with the repo's own host-built artifacts (compiled binaries, test
+# binaries, coverage output); those are per-repo and belong here because
+# a host build otherwise drops them into the context.
+
+# Repository metadata: exactly one, at the context root.
 .git
-node_modules
-.DS_Store
+
+# Environment and secrets. These are the reason the prefixes matter: a
+# developer's local copy is invisible to every git-based check.
+**/.env
+**/.env.*
+**/*.pem
+**/*.key
+
+# Dependencies: restored inside the image, never copied in.
+**/node_modules
+
+# OS metadata.
+**/.DS_Store
+**/Thumbs.db
+
+# Editor state. Never a build input, and it churns under a developer's
+# hands, so it invalidates COPY for reasons unrelated to the source.
+**/*.swp
+**/*.swo
+**/*~
+**/*.bak
+**/.idea
+**/.vscode
+**/*.sublime-*

TODO.md

@@ -21,6 +21,17 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-08-09: Closed the secret exposure in the canonical `.dockerignore`: a
+  developer's local `.env`, `*.pem` or `*.key` was reaching the Docker build
+  context under `COPY . .`, invisible to every git-based check because
+  `.gitignore` covers it. The patterns are written to `.dockerignore`'s own
+  `filepath.Match` semantics — `**/`-prefixed so they hold at every depth, which
+  also fixes nested `node_modules` — rather than transplanted from `.gitignore`,
+  whose unprefixed form protects only the repository root while reading as
+  solved. `REPO_POLICIES.md` and both repo checklists now state that asymmetry
+  and require verification by enumerating the image rather than by reading the
+  patterns. Verified with a probe image before, against the naive unprefixed
+  form, and after.
 - 2026-08-09: Made the pinned golangci-lint actually propagate: REPO_POLICIES.md
   now carries the canonical `script/bootstrap` snippet for Go repos, which
   installs when the installed version does not match the pin (the old

prompts/EXISTING_REPO_CHECKLIST.md

@@ -37,6 +37,13 @@ with your task.
       `CHECK_EPOCH` rule in `REPO_POLICIES.md`. Without them the check layer is
       served from cache on an unchanged tree and the build reports a green it
       never ran.
+- [ ] Every depth-independent pattern in `.dockerignore` carries a `**/` prefix;
+      only genuinely root-anchored entries such as `.git` are unprefixed, and
+      `.gitignore`'s patterns have not been transplanted unmodified.
+      `.dockerignore` anchors an unprefixed pattern at the context root, so the
+      transplanted form leaves `config/.env` and `certs/server.key` in the build
+      context while reading as solved — see the `.dockerignore` rule in
+      `REPO_POLICIES.md`.
 - [ ] Gitea Actions workflow in `.gitea/workflows/` runs `script/cibuild` on
       push — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`

prompts/NEW_REPO_CHECKLIST.md

@@ -52,6 +52,12 @@ Template files can be fetched from:
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md`
 - [ ] `Dockerfile` and `.dockerignore` — fetch `.dockerignore` from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`
+    - Extend `.dockerignore` with the repo's own host-built artifacts, giving
+      every depth-independent pattern a `**/` prefix. Do not transplant
+      `.gitignore`'s patterns: `.dockerignore` anchors an unprefixed pattern at
+      the context root, so the copied form leaves `config/.env` in the build
+      context while reading as solved. See the `.dockerignore` rule in
+      `REPO_POLICIES.md`.
     - All Dockerfiles must run `make check` as a build step, and every stage
       containing a check-running `RUN` must declare `ARG CHECK_EPOCH` with the
       `RUN [ -n "$CHECK_EPOCH" ] || exit 1` guard immediately below it — see the

prompts/REPO_POLICIES.md

@@ -331,7 +331,39 @@ style conventions are in separate documents:
   editor files (`.swp`, `*~`), language build artifacts, and `node_modules/`.
   Fetch the standard `.gitignore` from
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitignore` when setting up
-  a new repo.
+  a new repo. These patterns are written to `.gitignore`'s own semantics, in
+  which an unanchored pattern already matches at every depth. They are not a
+  `.dockerignore` and must not be transplanted into one unmodified — see the
+  next rule.
+
+- **`.dockerignore` does not use `.gitignore` semantics, and copying patterns
+  across unmodified leaves secrets in the build context.** Docker matches with
+  Go `filepath.Match`: `*` does not cross `/`, and a pattern without a leading
+  `**/` is anchored at the build-context root. A `.dockerignore` listing `.env`,
+  `*.pem` and `*.key` therefore excludes only the copies at the repository root;
+  `config/.env` and `certs/server.key` still reach the context and can land in
+  an image layer. That file is more dangerous than a short one with no secret
+  patterns at all, because it reads as solved and stops anyone looking. Give
+  every depth-independent pattern the `**/` prefix — `**/.env`, `**/.env.*`,
+  `**/*.pem`, `**/*.key`, `**/node_modules` — and leave only genuinely
+  root-anchored entries such as `.git` unprefixed. The inverse move is equally
+  wrong: never apply `**/` to `.gitignore`, where it is redundant and produces a
+  file that is wrong in a way that looks careful. Each file is written to its
+  own semantics; neither is derived from the other. Fetch the standard
+  `.dockerignore` from
+  `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore` and extend
+  it with the repo's own host-built artifacts — a host `make build` that leaves
+  a compiled binary in the repo root puts that binary in the build context,
+  where `.gitignore` hides it from every git-based check.
+
+- **Verify `.dockerignore` by enumerating the image, not by reading the
+  patterns.** Plant files at the root _and_ at least two directories deep, build
+  a probe image that does `COPY . .`, and list what actually landed
+  (`docker run --rm --entrypoint find IMAGE /app`). Reading the patterns and
+  agreeing they look right is exactly what lets the root-only form through. The
+  `transferring context` size is not a substitute: a nested secret is a few
+  bytes, and BuildKit transfers only the delta from the previous build, so the
+  reported size describes the transfer and not the contents of the image.
 
 - **No build artifacts in version control.** Code-derived data (compiled
   bundles, minified output, generated assets) must never be committed to the