CI: update GHA instances from Ubuntu 18.04 to 20.04

The release binaries are built using Ubuntu 18.04 in Docker on Ubuntu 20.04
for glibc compatibility reason (issue 7255).

Fix issue 7297

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/release.yml

@@ -11,7 +11,7 @@ env:
 jobs:
   check:
     name: Check Signed Tag
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     timeout-minutes: 5
     outputs:
       stringver: ${{ steps.contentrel.outputs.stringver }}
@@ -53,29 +53,29 @@ jobs:
 
   build:
     name: Build Release Binaries
-    runs-on: ubuntu-${{ matrix.ubuntu }}
+    runs-on: ubuntu-20.04
     needs: [check]
     timeout-minutes: 30
     strategy:
       matrix:
         include:
           # Choose an old release of Ubuntu to avoid glibc issue https://github.com/containerd/containerd/issues/7255
-          - ubuntu: 18.04
-            platform: linux/amd64
-          - ubuntu: 18.04
-            platform: linux/arm64
-          - ubuntu: 18.04
-            platform: linux/ppc64le
+          - dockerfile-ubuntu: 18.04
+            dockerfile-platform: linux/amd64
+          - dockerfile-ubuntu: 18.04
+            dockerfile-platform: linux/arm64
+          - dockerfile-ubuntu: 18.04
+            dockerfile-platform: linux/ppc64le
           # riscv64 isn't supported by Ubuntu 18.04
-          - ubuntu: 22.04
-            platform: linux/riscv64
-          - ubuntu: 18.04
-            platform: windows/amd64
+          - dockerfile-ubuntu: 22.04
+            dockerfile-platform: linux/riscv64
+          - dockerfile-ubuntu: 18.04
+            dockerfile-platform: windows/amd64
     steps:
       - name: Set env
         shell: bash
         env:
-          MOS: ubuntu-${{ matrix.ubuntu }}
+          MOS: ubuntu-20.04
         run: |
           releasever=${{ github.ref }}
           releasever="${releasever#refs/tags/}"
@@ -107,14 +107,14 @@ jobs:
             export PREFIX_LEN=12
             BUILD_ARGS="--build-arg GATEWAY --build-arg PREFIX_LEN"
           fi
-          docker buildx build ${cache} --build-arg RELEASE_VER --build-arg UBUNTU_VERSION=${{ matrix.ubuntu }} --build-arg GO_VERSION ${BUILD_ARGS} -f .github/workflows/release/Dockerfile --platform=${PLATFORM} -o releases/ .
+          docker buildx build ${cache} --build-arg RELEASE_VER --build-arg UBUNTU_VERSION=${{ matrix.dockerfile-ubuntu }} --build-arg GO_VERSION ${BUILD_ARGS} -f .github/workflows/release/Dockerfile --platform=${PLATFORM} -o releases/ .
           echo PLATFORM_CLEAN=${PLATFORM/\//-} >> $GITHUB_ENV
 
           # Remove symlinks since we don't want these in the release Artifacts
           find ./releases/ -maxdepth 1 -type l | xargs rm
         working-directory: src/github.com/containerd/containerd
         env:
-          PLATFORM: ${{ matrix.platform }}
+          PLATFORM: ${{ matrix.dockerfile-platform }}
       - name: Save Artifacts
         uses: actions/upload-artifact@v2
         with:
@@ -123,7 +123,7 @@ jobs:
 
   release:
     name: Create containerd Release
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
     needs: [build, check]
     steps: