Fix 1.16.2/Dockerfile parsing error
Fix 1.16.2/Dockerfile parsing error

Missing a `\` in on a line causing the building of the container to
fail.

```
dockerfile parse error line 48: unknown instruction: UNBOUND_SHA256=2E32F28382    0C24C51CA1DD8AFECFDB747C7385A137ABE865C99DB4B257403581
```

MatthewVance/unbound-docker