Allow loopback and base fs sizes set by env var

Set env vars to control sparse filesize.

```bash

DOCKER_LOOPBACK_META_SIZE
DOCKER_LOOPBACK_DATA_SIZE
DOCKER_BASE_FS_SIZE
```

Default Values
```bash
DOCKER_LOOPBACK_META_SIZE=2 * 1024 * 1024 * 1024
DOCKER_LOOPBACK_DATA_SIZE=100 * 1024 * 1024 * 1024
DOCKER_BASE_FS_SIZE=10 * 1024 * 1024 * 1024

```