Switch to github actions for CI

.github/workflows/CI.yaml

@@ -0,0 +1,30 @@
+name: CI
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+jobs:
+  gofmt:
+    runs-on: ubuntu-latest
+    container: docker://golang:1.13
+    steps:
+    - uses: actions/checkout@v1
+    - name: run gofmt
+      run: test -z "$(gofmt -d . | tee /dev/stderr)"
+  gazelle:
+    runs-on: ubuntu-latest
+    container: docker://l.gcr.io/google/bazel:2.1.0
+    steps:
+    - uses: actions/checkout@v1
+    - name: run gazelle
+      run: bazel run :gazelle && git diff --exit-code HEAD --
+  build:
+    runs-on: ubuntu-latest
+    container: docker://l.gcr.io/google/bazel:2.1.0
+    steps:
+    - uses: actions/checkout@v1
+    - name: build
+      run: bazel build //...

README.md

@@ -1,4 +1,4 @@
-# Buildbarn Browser [![Build status](https://badge.buildkite.com/e3a50ce641ac264df8fffe8c52931f850f8e00e8852a5c7ee4.svg)](https://buildkite.com/buildbarn/bb-browser)
+# Buildbarn Browser [![Build status](https://github.com/buildbarn/bb-browser/workflows/CI/badge.svg)](https://github.com/buildbarn/bb-browser/actions)
 
 Buildbarn Browser is a simple web service written in Go that can display
 objects stored in a Content Addressable Storage (CAS) and an Action