feat: use gha cache exporter when building via buildkit

Buildkit builds expose custom cache exporters which can be used to locally speed up builds on Github Actions. The gha cacher will use the Github Actions cache api instead of the remote registry for caching.

To enable this, set the following two environment variables.

```
DOCKER_BUILDKIT=1
BUILDKIT_CACHE_EXPORTER=gha
```

Note that the cache mode is set to min by default (no value == min) due to a potential timeout issue within the exporter. See https://github.com/moby/buildkit/issues/2276 for details. To switch the cache mode, set the following environment variable:

```
BUILDKIT_CACHE_MODE=max
```

cli/cmd/docker/builder.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net/http"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -37,7 +38,9 @@ type BuildOpts struct {
 	LogFile *os.File
 }
 
-// BuildLocal
+// BuildLocal builds the image via docker
+// If the DOCKER_BUILDKIT environment variable is set, builds will switch to
+// using the docker binary directly (with buildkit enabled)
 func (a *Agent) BuildLocal(ctx context.Context, opts *BuildOpts) (err error) {
 	if os.Getenv("DOCKER_BUILDKIT") == "1" {
 		return buildLocalWithBuildkit(ctx, *opts)
@@ -178,6 +181,7 @@ func AddDockerfileToBuildContext(dockerfileCtx io.ReadCloser, buildCtx io.ReadCl
 }
 
 func buildLocalWithBuildkit(ctx context.Context, opts BuildOpts) error {
+	fmt.Println("Triggering build via buildkit")
 	if _, err := exec.LookPath("docker"); err != nil {
 		return fmt.Errorf("unable to find docker binary in PATH for buildkit build: %w", err)
 	}
@@ -203,12 +207,35 @@ func buildLocalWithBuildkit(ctx context.Context, opts BuildOpts) error {
 		extraDockerArgs = parsedFields
 	}
 
+	cacheFrom := fmt.Sprintf("%s:%s", opts.ImageRepo, opts.CurrentTag)
+	cacheTo := ""
+	if ok, _ := isRunningInGithubActions(); ok && os.Getenv("BUILDKIT_CACHE_EXPORTER") == "gha" {
+		fmt.Println("Github Actions environment detected, switching to the GitHub Actions cache exporter")
+		cacheFrom = "type=gha"
+		cacheTo = "type=gha"
+
+		// CacheMode is set separately to avoid cases where builds may timeout for
+		// dockerfiles with many layers.
+		// See https://github.com/moby/buildkit/issues/2276 for details.
+		cacheMode := os.Getenv("BUILDKIT_CACHE_MODE")
+		if cacheMode == "min" || cacheMode == "max" {
+			fmt.Printf("Setting GHA cache mode to %s\n", cacheMode)
+			cacheTo = fmt.Sprintf("type=gha,mode=%s", cacheMode)
+		} else if cacheMode != "" {
+			return errors.New("error while parsing buildkit environment variables: BUILDKIT_CACHE_MODE set to invalid value, valid values: min, max")
+		}
+	}
+
 	commandArgs := []string{
 		"build",
 		"-f", dockerfileName,
 		"--tag", fmt.Sprintf("%s:%s", opts.ImageRepo, opts.Tag),
-		"--cache-from", fmt.Sprintf("%s:%s", opts.ImageRepo, opts.CurrentTag),
+		"--cache-from", cacheFrom,
 	}
+	if cacheTo != "" {
+		commandArgs = append(commandArgs, "--cache-to", cacheTo)
+	}
+
 	for key, val := range opts.Env {
 		commandArgs = append(commandArgs, "--build-arg", fmt.Sprintf("%s=%s", key, val))
 	}
@@ -313,3 +340,33 @@ func sliceContainsString(haystack []string, needle string) bool {
 
 	return false
 }
+
+// isRunningInGithubActions detects if the environment is a github actions
+// runner environment by validating certain environment variables and then
+// making a call to the Github api to verify the run itself.
+func isRunningInGithubActions() (bool, error) {
+	for _, key := range []string{"CI", "GITHUB_RUN_ID", "GITHUB_TOKEN", "GITHUB_REPOSITORY"} {
+		if key == "" {
+			return false, nil
+		}
+	}
+
+	url := fmt.Sprintf("https://api.github.com/repos/%s/actions/runs/%s", os.Getenv("GITHUB_REPOSITORY"), os.Getenv("GITHUB_RUN_ID"))
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err == nil {
+		return false, err
+	}
+	req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", os.Getenv("GITHUB_TOKEN")))
+
+	client := http.Client{
+		Timeout: 5 * time.Second,
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return false, err
+	}
+	defer resp.Body.Close() //nolint:errcheck
+
+	return resp.StatusCode == http.StatusOK, nil
+}