i#7284: Bootstrap Alpine build environment from minirootfs in workflow (#7289)

Previously, the build environment is bootstrapped with a static version
of apk downloaded from Alpine mirror, whose URL isn't stable since the
package is upgraded irregularly.

This commit switches to prebuilt minirootfs tarball, which is considered
as release artifacts and has a stable URL, saving the burden of
frequently synchronizing with Alpine upstream.

Fixes: #7284

Author: ziyao233

.github/workflows/ci-x86.yml

@@ -264,30 +264,30 @@ jobs:
     # Create an Alpine Linux 3.21.2 chroot environment for building
     - name: Create Build Environment
       run: |
-        curl -L https://dl-cdn.alpinelinux.org/v3.21/main/x86_64/apk-tools-static-2.14.6-r2.apk | \
-        tar xzv sbin/apk.static
+        sudo mkdir alpine-root
+        curl -L https://dl-cdn.alpinelinux.org/v3.21/releases/x86_64/alpine-minirootfs-3.21.3-x86_64.tar.gz | \
+        sudo tar xzv -C alpine-root
+
+        cd alpine-root
+        sudo mount -o bind /dev/ dev
+        sudo mount -t proc proc proc
+        sudo mount -t sysfs sys sys
+        sudo mount -o bind ${{ github.workspace }}/DynamoRIO root
+        # Use host DNS configuration in the chroot environment
+        sudo cp /etc/resolv.conf etc/
+
+        sudo chroot . sbin/apk upgrade
 
-        sudo mkdir -p alpine-root/etc/apk
-        echo "https://dl-cdn.alpinelinux.org/v3.21/main/" | \
-        sudo tee alpine-root/etc/apk/repositories
         # Meta-package build-base simplifies dependencies, but it pulls in
         # fortify-headers. Alpine ships an out-of-date version whose headers
         # contain unnecessary includes, causing naming conflicts.
         # TODO i#1973: simplify dependency with build-base after fixing
         # the compatibility problem.
-        # Supply --allow-untrusted since we're creating a rootfs from scratch
-        # thus don't have any public key available.
-        sudo sbin/apk.static -p alpine-root -U --initdb --allow-untrusted add \
+        sudo chroot . sbin/apk add \
           alpine-base binutils bash cmake doxygen file g++ git libunwind-dev \
           lz4-dev lz4-static linux-headers make musl-dev musl-libintl perl \
           patch xz-dev zlib-dev zlib-static
 
-        cd alpine-root
-        sudo mount -o bind /dev/ dev
-        sudo mount -t proc proc proc
-        sudo mount -t sysfs sys sys
-        sudo mount -o bind ${{ github.workspace }}/DynamoRIO root
-
     - name: Run Suite
       working-directory: ${{ github.workspace }}
       run: |