build: run the browser e2e suites in CI (closes #259)

Nothing automatic ran either e2e suite, so every browser-level guarantee in this
wallet -- WebAssembly under the shipped CSP, the recovery-phrase and private-key
DOM wipes, the ConfirmTx spend gate, the dApp approval round trips -- held only
when a human or an agent remembered to run it by hand.

.gitea/workflows/e2e.yml adds two jobs, e2e-chrome and e2e-firefox, one per
browser so a Chrome failure cannot hide the Firefox result. They are separate
from the check workflow: REPO_POLICIES.md caps make test at 20 seconds and
script/cibuild is a docker build whose Dockerfile runs make check, so neither the
cap nor the local fast path is touched. make check is byte-for-byte unchanged.

Neither suite could run on the runner as it stood, and the reason is not
docker-in-docker. The runner executes a job inside a container against the HOST's
docker daemon, and the job's checkout lives on a docker volume rather than a host
path, so `docker run -v "$PWD:/work"` is resolved by the host, silently succeeds
and mounts an empty directory -- measured on this runner. The runner image's node
is also too old to install this repo's dependencies. Both suites therefore ship
the repo to the daemon as a build context and build the extension inside the
pinned image, which leaves docker as the only prerequisite on a runner or a
laptop. The suites themselves are unchanged; only how the repo reaches the
container is.

Both scripts now build with --iidfile and run the image by ID rather than by tag,
so two clones running a suite at once on the same host cannot swap it under each
other.

The jobs report, they do not gate. Whether a check blocks a merge is Gitea branch
protection, which this repo does not configure, so a failure is a red mark a
reviewer must account for. Nothing can pass vacuously: no continue-on-error, no
`|| true`, and both scripts exit non-zero when docker is missing, when the image
build fails and when the browser fails to start.

Wiring this up measured something that has to be said rather than absorbed: the
Chrome suite is flaky under load. Two of six runs of unmutated code on a loaded
machine lost the approval popup out from under the dApp signing wait. It is
filed as #287 and not papered over here -- no retry wrapper, no longer timeout,
no weakened assertion -- and it is the reason e2e-chrome cannot become a
required check yet. README and the workflow say so where a reader meets them.

script/test-e2e-firefox

@@ -5,12 +5,17 @@
 #
 # Deliberately NOT called by script/check or script/test, for the same
 # reason as the Chrome suite: REPO_POLICIES.md caps make test at 20 seconds
-# and a browser suite does not fit.
+# and a browser suite does not fit. .gitea/workflows/e2e.yml also runs it
+# on every push, in a job separate from check.
 #
-# Unlike script/test-e2e this builds its image locally, because no
+# Unlike script/test-e2e this builds its base image locally, because no
 # published image carries both a pinned Firefox and a matching geckodriver.
 # All three external artifacts are pinned by digest inside the Dockerfile;
-# see tests/e2e/firefox/Dockerfile.
+# see tests/e2e/firefox/Dockerfile, which also explains why the repo and
+# the extension build are baked into the image rather than mounted.
+#
+# Docker is the only prerequisite: nothing here depends on the node, yarn
+# or make on the machine that starts the run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -18,6 +23,14 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 IMAGE="$("$SCRIPT_DIR/projectname")-e2e-firefox"
 
+IIDFILE=""
+
+cleanup() {
+    if [ -n "$IIDFILE" ]; then
+        rm -f "$IIDFILE"
+    fi
+}
+
 main() {
     cd "$ROOT"
 
@@ -26,16 +39,20 @@ main() {
         exit 1
     fi
 
-    echo "Building extension for e2e..."
-    yarn run build 2>&1
+    IIDFILE="$(mktemp)"
+    trap cleanup EXIT
+    trap 'cleanup; exit 130' INT TERM
 
-    # The build context is tests/e2e/firefox/ and holds nothing but the
-    # Dockerfile: the harness itself arrives over the bind mount below, so
-    # editing it never invalidates an image layer.
-    echo "Building the pinned Firefox e2e image..."
-    docker build -t "$IMAGE" "$ROOT/tests/e2e/firefox"
+    echo "Building the pinned Firefox e2e image (extension included)..."
+    docker build --iidfile "$IIDFILE" -t "$IMAGE" \
+        -f tests/e2e/firefox/Dockerfile .
 
     echo "Running the Firefox e2e suite..."
+    # The image is run by ID, not by tag: where two clones of this repo run
+    # the suite at once, the other build can move the tag between this
+    # build and this run, and the suite would then silently test the other
+    # checkout.
+    #
     # --shm-size=1g: Firefox needs more than the default 64MB /dev/shm.
     # --network none: the suite stubs nothing, so this is what keeps the
     #   run offline and deterministic. The extension swallows its own
@@ -43,8 +60,8 @@ main() {
     #   network note in README.md. Weaker than the Chrome suite's
     #   fixture interception, and honestly so — it proves no request
     #   escaped, but it cannot report which ones were attempted.
-    # --user: keep files the suite touches owned by the caller, not root.
-    # HOME=/tmp: the mapped uid has no home directory in the image.
+    # HOME=/tmp: the image's root home is not a reliable place for the
+    #   browser profile.
     #
     # No --privileged. Firefox's sandbox logs
     # "CanCreateUserNamespace() clone() failure: EPERM" on startup here;
@@ -52,11 +69,8 @@ main() {
     docker run --rm \
         --shm-size=1g \
         --network none \
-        --user "$(id -u):$(id -g)" \
         -e HOME=/tmp \
-        -v "$ROOT:/work" \
-        -w /work \
-        "$IMAGE" \
+        "$(cat "$IIDFILE")" \
         node tests/e2e/firefox/run.js dist/firefox
 }