Add simplified badge and Build workflow.

PiperOrigin-RevId: 312159017

.github/workflows/build.yml

@@ -0,0 +1,21 @@
+name: "Build"
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  default:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/cache@v1
+      with:
+        path: ~/.cache/bazel
+        key: ${{ runner.os }}-bazel-${{ hashFiles('WORKSPACE') }}
+        restore-keys: |
+          ${{ runner.os }}-bazel-
+    - run: make

README.md

@@ -1,7 +1,6 @@
 ![gVisor](g3doc/logo.png)
 
-[![Status](https://storage.googleapis.com/gvisor-build-badges/build.svg)](https://storage.googleapis.com/gvisor-build-badges/build.html)
-[![gVisor chat](https://badges.gitter.im/gvisor/community.png)](https://gitter.im/gvisor/community)
+![](https://github.com/google/gvisor/workflows/Build/badge.svg)
 
 ## What is gVisor?