go.mod, .github/workflows: bump Go version, add start of CI

Updates #27

Author: bradfitz

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+name: Go CI
+
+on:
+  push:
+    branches:
+      - "main"
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+
+      - uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
+        with:
+          go-version-file: go.mod
+
+      - name: build
+        run: go install
+
+      - name: test
+        run: go test ./...

go.mod

@@ -1,7 +1,7 @@
 module github.com/tailscale/mkctr
 
 // Use point versions of Go, see https://github.com/tailscale/tailscale/pull/13485
-go 1.23.1
+go 1.25.5
 
 require github.com/google/go-containerregistry v0.20.2