Configure a build matrix for Clang using containers.

Change-Id: I8dbc53f3a45f1f1f215e8c949275b85fdef2c8e3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57800
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d51fb5c..2108ce0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,7 +3,7 @@
   push:
     branches: [master]
 jobs:
-  build-clang:
+  build:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
@@ -15,6 +15,19 @@
     - uses: actions/checkout@v2
     - run: make && make test
       shell: bash
+  build-clang:
+    runs-on: ubuntu-latest
+    container: ericwf/llvm:${{ matrix.tag }}
+    strategy:
+      matrix:
+        tag: [4.x, 5.x, 6.x, 7.x, 8.x, 9.x, 10.x, 11.x]
+    env:
+      CC: clang
+      CXX: clang++
+    steps:
+    - uses: actions/checkout@v2
+    - run: make && make test
+      shell: bash
   build-gcc:
     runs-on: ubuntu-latest
     container: gcc:${{ matrix.tag }}