Default ´dockerfile´ value is wrong

[JoelLinn]

The default value for the -f flag of docker build is PATH/Dockerfile.
However, this plugin uses just Dockerfile as default:

drone-docker/cmd/drone-docker/main.go

Lines 111 to 115 in f08821b

	cli.StringFlag{
	Name: "dockerfile",
	Usage: "build dockerfile",
	Value: "Dockerfile",
	EnvVar: "PLUGIN_DOCKERFILE",

For this reason one needs to explicitly set the dockerfile when using a non default context (PATH), for example:

settings:
  context: 5.6/buster/cli
  dockerfile: 5.6/buster/cli/Dockerfile

Otherwise, this error will be thrown:

unable to prepare context: unable to evaluate symlinks in Dockerfile path: lstat /drone/src/Dockerfile: no such file or directory