CI: use docker cp instead of bind mounts (sibling-container path trap)

Under act_runner, inner 'docker run' hits the host daemon via the mounted
socket, so -v "$PWD/test:/work" mounts a nonexistent host path (empty).
Stream files in/out with docker cp instead. Same pattern will drive the
GlueXML release workflow.

.gitea/workflows/ci.yml

@@ -24,27 +24,37 @@ jobs:
       - name: Build image
         run: docker build -t mpq-packager:ci .
 
+      # NOTE: act_runner runs these `docker` commands against the HOST daemon via
+      # the mounted socket, so bind-mounting the job's files (-v "$PWD:/work")
+      # doesn't work — the source path isn't on the host. Use `docker cp` to
+      # stream files in/out of a created container instead. The GlueXML release
+      # workflow uses this same pattern.
       - name: Package the fixture
         run: |
           set -euo pipefail
-          docker run --rm \
+          cid=$(docker create \
             -e INPUT_MANIFEST=mpq.yaml \
             -e INPUT_VERSION=v0.0.0-ci \
             -e INPUT_OUT_DIR=/work/dist \
             -e GITHUB_WORKSPACE=/work \
-            -v "$PWD/test:/work" \
-            mpq-packager:ci
+            mpq-packager:ci)
+          docker cp test/. "$cid:/work"
+          docker start -a "$cid"
+          docker cp "$cid:/work/dist/patch-Z.mpq" ./patch-Z.mpq
+          docker rm "$cid" >/dev/null
 
       - name: Assert archive is valid and contains the expected file
         run: |
           set -euo pipefail
-          test -s test/dist/patch-Z.mpq
-          echo "size: $(stat -c%s test/dist/patch-Z.mpq) bytes"
-          # Read the archive back with StormLib (mpqpack --list/--cat).
-          docker run --rm --entrypoint mpqpack -v "$PWD/test:/work" mpq-packager:ci \
-            --list /work/dist/patch-Z.mpq | tee /tmp/list.txt
-          grep -q 'Interface\\AddOns\\Fixture\\Hello.lua' /tmp/list.txt
-          docker run --rm --entrypoint mpqpack -v "$PWD/test:/work" mpq-packager:ci \
-            --cat /work/dist/patch-Z.mpq 'Interface\AddOns\Fixture\Hello.lua' \
+          test -s patch-Z.mpq
+          echo "size: $(stat -c%s patch-Z.mpq) bytes"
+          # Read the archive back with StormLib (mpqpack --list/--cat) inside a
+          # sleeping container we exec into.
+          cid=$(docker run -d --entrypoint sleep mpq-packager:ci 120)
+          docker cp patch-Z.mpq "$cid:/patch-Z.mpq"
+          docker exec "$cid" mpqpack --list /patch-Z.mpq | tee list.txt
+          grep -q 'Interface\\AddOns\\Fixture\\Hello.lua' list.txt
+          docker exec "$cid" mpqpack --cat /patch-Z.mpq 'Interface\AddOns\Fixture\Hello.lua' \
             | grep -q 'VERSION = "v0.0.0-ci"'
+          docker rm -f "$cid" >/dev/null
           echo "OK: archive valid, path + substitution confirmed"