Add Alpine to github actions CI

Restores Alpine CI as removed in these commits:

- ff0afd14c2 (Remove Cirrus CI, 2026-05-28)
- f48166ff36 (update-dependencies: remove things used for Cirrus, 2026-05-29)
- dc9668c8a4 (Remove unused workflow to build docker images, 2026-05-30)

See following commits for the rest.

Part of #12626

.github/workflows/test.yml

@@ -10,6 +10,13 @@ permissions:
   contents: read
 
 jobs:
+  alpine:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2, build_tools/update-dependencies.sh
+    - run:
+        docker/docker_run_tests.sh docker/alpine.Dockerfile
+
   ubuntu:
     runs-on: ubuntu-latest
     steps:

docker/alpine.Dockerfile

@@ -0,0 +1,45 @@
+# Version set by updatecli.d/docker.yml
+FROM alpine:3.23
+LABEL org.opencontainers.image.source=https://github.com/fish-shell/fish-shell
+
+ENV LANG=C.UTF-8
+ENV LC_ALL=C.UTF-8
+
+RUN apk add --no-cache \
+    bash \
+    cargo \
+    g++ \
+    gettext-dev \
+    git \
+    libintl \
+    musl-dev \
+    pcre2-dev \
+    py3-pexpect \
+    py3-pip \
+    python3 \
+    rust \
+    sudo \
+    tmux
+
+RUN addgroup -g 1000 fishuser
+
+RUN adduser \
+    --disabled-password \
+    --gecos "" \
+    --home "/home/fishuser" \
+    --ingroup fishuser \
+    --uid 1000 \
+    fishuser
+
+RUN mkdir -p /home/fishuser/fish-build \
+    && mkdir /fish-source \
+    && chown -R fishuser:fishuser /home/fishuser /fish-source
+
+USER fishuser
+WORKDIR /home/fishuser
+
+COPY fish_run_tests.sh /
+
+ENV FISH_CHECK_LINT=false
+
+CMD /fish_run_tests.sh

docker/docker_run_tests.sh

@@ -54,8 +54,9 @@ docker build \
     "$workspace_root"/docker/context/
 
 # Run tests in it, allowing them to fail without failing this script.
+# shellcheck disable=SC2046  # for the isatty snippet
 # shellcheck disable=SC2086  # $DOCKER_EXTRA_ARGS should have globbing and splitting applied.
-docker run -it \
+docker run -i $(isatty 0 && printf %s -t) \
     --mount type=bind,source="$workspace_root",target=/fish-source,readonly \
     $DOCKER_EXTRA_ARGS \
     "$IMG_TAGNAME"

updatecli.d/docker.yml

@@ -1,6 +1,13 @@
 name: "Update Linux docker images"
 
 sources:
+  alpine_latest_version:
+    kind: dockerimage
+    spec:
+      image: alpine
+      tagfilter: "^[0-9]+\\.[0-9]+$"
+      versionfilter:
+        kind: semver
   ubuntu_latest_lts_version:
     kind: shell
     spec:
@@ -17,6 +24,14 @@ sources:
           tail -1
 
 targets:
+  update_alpine_dockerfile:
+    name: "Update Alpine Linux version"
+    sourceid: alpine_latest_version
+    kind: file
+    spec:
+      file: docker/alpine.Dockerfile
+      matchpattern: "FROM alpine:.*"
+      replacepattern: 'FROM alpine:{{ source "alpine_latest_version" }}'
   ubuntu_latest_version_dockerfile:
     name: "Update oldest non-EOL Ubuntu Linux version"
     sourceid: ubuntu_latest_lts_version