add a build cache example to the README

With a test-cache job on the repo's workflow, to show how it works.
We add a third party module dependency just to show what modules are
downloaded at each run.

The GOPATH example in the README was also slightly wrong, as it did not
set up the working directory inside GOPATH for the future steps. Fix
that.

Remove test-gopath from this repository's workflow, as having to
download dependencies via 'go get' would make it non-reproducible and
perhaps fail in the future. The example in the README is enough moving
forward, as everyone should be using modules by now.

Fixes #7.

Author: mvdan

.github/workflows/test.yml

@@ -17,10 +17,7 @@ jobs:
     - name: Test
       run: go test ./...
 
-  test-gopath:
-    env:
-      GOPATH: ${{ github.workspace }}
-      GO111MODULE: off
+  test-cache:
     runs-on: ubuntu-latest
     steps:
     - name: Install Go
@@ -29,7 +26,15 @@ jobs:
         go-version: 1.15.x
     - name: Checkout code
       uses: actions/checkout@v2
+    - uses: actions/cache@v2
       with:
-        path: ./src/github.com/${{ github.repository }}
+        path: |
+          ~/go/pkg/mod              # Module download cache
+          ~/.cache/go-build         # Build cache (Linux)
+          ~/Library/Caches/go-build # Build cache (Mac)
+          '%LocalAppData%\go-build' # Build cache (Windows)
+        key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+        restore-keys: |
+          ${{ runner.os }}-go-
     - name: Test
       run: go test ./...

README.md

@@ -87,6 +87,26 @@ downloaded modules:
       ${{ runner.os }}-go-
 ```
 
+You can also include Go's build cache, to improve incremental builds:
+
+```yaml
+- uses: actions/cache@v2
+  with:
+    path: |
+      ~/go/pkg/mod              # Module download cache
+      ~/.cache/go-build         # Build cache (Linux)
+      ~/Library/Caches/go-build # Build cache (Mac)
+      '%LocalAppData%\go-build' # Build cache (Windows)
+    key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+    restore-keys: |
+      ${{ runner.os }}-go-
+```
+
+This is demonstrated via the `test-cache` job [in this very repository](https://github.com/mvdan/github-actions-golang/actions).
+
+See [this guide](https://docs.github.com/en/free-pro-team@latest/actions/guides/caching-dependencies-to-speed-up-workflows)
+for more details.
+
 #### How do I run a step conditionally?
 
 You can use `if` conditionals, using their [custom expression
@@ -168,11 +188,14 @@ jobs:
     env:
       GOPATH: ${{ github.workspace }}
       GO111MODULE: off
+    defaults:
+      run:
+        working-directory: ${{ env.GOPATH }}/src/github.com/${{ github.repository }}
     steps:
     - name: Checkout code
       uses: actions/checkout@v2
       with:
-        path: ./src/github.com/${{ github.repository }}
+        path: ${{ env.GOPATH }}/src/github.com/${{ github.repository }}
 ```
 
 ## Quick links

actions.go

@@ -6,10 +6,14 @@ package actions
 import (
 	"fmt"
 	"runtime"
+
+	"rsc.io/quote"
 )
 
 func Demo() {
 	fmt.Printf("Go version: %s\n", runtime.Version())
 	fmt.Printf("GOOS: %s\n", runtime.GOOS)
 	fmt.Printf("GOARCH: %s\n", runtime.GOARCH)
+
+	fmt.Println(quote.Go())
 }

go.mod

@@ -1,3 +1,5 @@
 module dummy.module/actions
 
 go 1.14
+
+require rsc.io/quote v1.5.2

go.sum

@@ -0,0 +1,6 @@
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:qgOY6WgZOaTkIIMiVjBQcw93ERBE4m30iBm00nkL0i8=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:w5fcysjrx7yqtD/aO+QwRjYZOKnaM9Uh2b40tElTs3Y=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:7uVkIFmeBqHfdjD+gZwtXXI+RODJ2Wc4O7MPEh/QiW4=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=