ci: Use Github Actions to run integration tests and push Docker images

We're running against the usage limits in Gitlab CI (500), and Github
Actions should have more (2000).

So this patch replaces Gitlab CI with Github actions for running
integration tests, and build and push Docker images (to Dockerhub and
Gitlab registry).

We'll see how the usage levels are in a few months.

.github/workflows/gotests.yml

@@ -0,0 +1,39 @@
+name: "gotests"
+
+on:
+  push:
+    branches: [ "master", "next" ]
+  pull_request:
+    # The branches below must be a subset of the branches above
+    branches: [ "master", "next" ]
+  schedule:
+    - cron: '29 21 * * 6'
+
+jobs:
+  oldest_supported:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-go@v3
+        with:
+          go-version-file: 'go.mod'
+      - name: normal tests
+        run: go test ./...
+      - name: race tests
+        run: go test -race ./...
+
+  latest:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-go@v3
+        with:
+          go-version: "1.x"
+          check-latest: true
+      - name: normal tests
+        run: go test ./...
+      - name: race tests
+        run: go test -race ./...
+