build: mount build cache directories

so `$ bob build` (when ran on host) can be faster on subsequent invocations

Author: joonas-fi

cmd/bob/build.go

@@ -72,6 +72,14 @@ func runBuilder(builder bobfile.BuilderSpec, buildCtx *BuildContext, opDesc stri
 		"--volume", "/tmp/build:/tmp/build", // cannot map to /tmp because at least apt won't work (permission issues?)
 	}
 
+	baseImageConf, err := loadNonOptionalBaseImageConf(builder)
+	if err == nil { // it's optional here. used to mount the cache directories
+		for _, pathContainer := range baseImageConf.PathsToCache {
+			pathHostSide := makeCachePathHostSide(pathContainer)
+			buildArgs = append(buildArgs, fmt.Sprintf("--mount=type=bind,source=%s,destination=%s", pathHostSide, pathContainer))
+		}
+	}
+
 	if builder.Workdir != "" {
 		buildArgs = append(buildArgs, "--workdir", builder.Workdir)
 	}

cmd/bob/devshim.go

@@ -221,8 +221,7 @@ func makeCacheDir(dir string) error {
 		return err
 	}
 
-	// "/go/pkg" => "/tmp/build/go/pkg"
-	cacheCounterpart := filepath.Join("/tmp/build/", dir)
+	cacheCounterpart := makeCachePathHostSide(dir)
 
 	if err := os.MkdirAll(cacheCounterpart, 0755); err != nil {
 		return err
@@ -231,6 +230,11 @@ func makeCacheDir(dir string) error {
 	return os.Symlink(cacheCounterpart, dir)
 }
 
+// "/go/pkg" => "/tmp/build/go/pkg" (at host-side)
+func makeCachePathHostSide(pathInContainer string) string {
+	return filepath.Join("/tmp/build/", pathInContainer)
+}
+
 func readShimConfig() (*shimConfig, error) {
 	shimConf := &shimConfig{}
 	return shimConf, jsonfile.ReadDisallowUnknownFields(filepath.Join(shimDataDirContainer, shimConfigFile), shimConf)