Fix config test type mismatch

I run test, but I see below error.

```
go test ./...
# github.com/mackerelio/mackerel-agent/command
command/command_test.go:241: cannot use conf (type config.Config) as type *config.Config in argument to loop
ok  	github.com/mackerelio/mackerel-agent	0.014s
?   	github.com/mackerelio/mackerel-agent/agent	[no test files]
FAIL	github.com/mackerelio/mackerel-agent/command [build failed]
ok  	github.com/mackerelio/mackerel-agent/config	0.024s
ok  	github.com/mackerelio/mackerel-agent/logging	0.010s
ok  	github.com/mackerelio/mackerel-agent/mackerel	0.025s
?   	github.com/mackerelio/mackerel-agent/metrics	[no test files]
ok  	github.com/mackerelio/mackerel-agent/metrics/linux	4.074s
ok  	github.com/mackerelio/mackerel-agent/spec	0.005s
ok  	github.com/mackerelio/mackerel-agent/spec/linux	0.060s
ok  	github.com/mackerelio/mackerel-agent/util/linux	0.006s
?   	github.com/mackerelio/mackerel-agent/version	[no test files]
```

I guess because of #36.
I fixed type mismatch of the test code.

And  I add ```Dockerfile``` to check easily on local machine.