ci: fix not found package when check sha256

.github/workflows/build_packages.yaml

@@ -308,6 +308,7 @@ jobs:
                    make $PROFILE-pkg || cat rebar3.crashdump; \
                    EMQX_NAME=$PROFILE && .ci/build_packages/tests.sh"
     - name: create sha256
+      working-directory: source
       env:
         PROFILE: ${{ matrix.profile}}
       run: |
@@ -358,7 +359,6 @@ jobs:
           type=ref,event=pr
           type=ref,event=tag
           type=semver,pattern={{version}}
-          type=semver,pattern={{major}}.{{minor}}
         labels:
           org.opencontainers.image.otp.version=${{ matrix.otp }}
     - uses: docker/login-action@v1
@@ -368,7 +368,7 @@ jobs:
         password: ${{ secrets.DOCKER_HUB_TOKEN }}
     - uses: docker/build-push-action@v2
       with:
-        push: ${{ github.event_name == 'release' }}
+        push: ${{ github.event_name == 'release' && !github.event.release.prerelease }}
         pull: true
         no-cache: true
         platforms: linux/amd64,linux/arm64