feat: allow setting DOCKER_BIN path for docker execution

This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.

plugins/repo/src/subcommands/purge-cache/purge-cache.go

@@ -25,7 +25,9 @@ func main() {
 	dokkuGlobalRunArgs := common.MustGetEnv("DOKKU_GLOBAL_RUN_ARGS")
 	image := common.GetDeployingAppImageName(appName, "", "")
 	if info, _ := os.Stat(cacheDir); info != nil && info.IsDir() {
-		purgeCacheCmd := common.NewShellCmd(strings.Join([]string{"docker run --rm", dokkuGlobalRunArgs,
+		purgeCacheCmd := common.NewShellCmd(strings.Join([]string{
+			common.DockerBin(),
+			"run --rm", dokkuGlobalRunArgs,
 			"-v", strings.Join([]string{cacheHostDir, ":/cache"}, ""), image,
 			`find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} ;`}, " "))
 		purgeCacheCmd.Execute()