GHA: Switch to Docker image for Alpine workflow (ldc-developers#4705)

.github/workflows/alpine_musl.yml

@@ -11,20 +11,17 @@ jobs:
   test:
     name: Alpine Linux (musl libc)
     runs-on: ubuntu-latest
+    container: alpine:3.20
     steps:
+      - name: Install prerequisites
+        run: apk add ldc git g++ cmake ninja llvm-dev llvm-static compiler-rt libxml2-static zstd-static zlib-static bash grep diffutils make
+
       - uses: actions/checkout@v4
         with:
           submodules: true
           fetch-depth: 50
 
-      - name: Setup latest Alpine Linux
-        uses: jirutka/setup-alpine@v1
-        with:
-          branch: v3.20
-          packages: ldc git g++ cmake ninja llvm-dev llvm-static compiler-rt libxml2-static zstd-static zlib-static bash grep diffutils make
-
       - name: Build LDC bootstrap
-        shell: alpine.sh {0}
         run: |
           ninja --version
           set -eux
@@ -43,7 +40,6 @@ jobs:
       # TODO: Add '-DLLVM_IS_SHARED=OFF' when static linking is fully supported
       # TSan and XRay do not work.
       - name: Build LDC & LDC D unittests & defaultlib unittest runners
-        shell: alpine.sh {0}
         run: |
           set -eux
           cmake -G Ninja . \
@@ -60,19 +56,16 @@ jobs:
 
       - name: Run LDC D unittests
         if: success() || failure()
-        shell: alpine.sh {0}
         run: ctest --output-on-failure -R "ldc2-unittest"
 
       - name: Run LIT testsuite
         if: success() || failure()
-        shell: alpine.sh {0}
         run: |
           set -eux
           ctest -V -R "lit-tests"
 
       - name: Run DMD testsuite
         if: success() || failure()
-        shell: alpine.sh {0}
         run: |
           # These two tests require extra flags "-link-defaultlib-debug -frame-pointer=all", https://github.com/ldc-developers/ldc/issues/4694.
           # Run them separately with these flags, and then remove them before running the full testsuite.
@@ -86,7 +79,6 @@ jobs:
 
       - name: Run defaultlib unittests & druntime integration tests
         if: success() || failure()
-        shell: alpine.sh {0}
         run: |
           set -eux
           ctest -j$(nproc) --output-on-failure -E "dmd-testsuite|lit-tests|ldc2-unittest"