Detect value set previously by ENV

If a Dockerfile contains a env variable set and then used, the analyzer should get the actual value and analyze it.
```
ENV HTTP_PORT=8080

EXPOSE $HTTP_PORT
```

Now it prints this output
```
1 - strconv.Atoi: parsing "$HTTP_PORT": invalid syntax

2 - port 0 exposed at line 30 could be wrong. TCP/IP port numbers below 1024 are privileged port numbers
```