Update Dockerfile

Fixed an issue brought up by the docker cli during build.

```
1 warning found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 25)
```

Author: frob

Dockerfile

@@ -20,7 +20,7 @@
 # /data # exit           // exit container
 
 # Start from the latest golang base image
-FROM golang:latest as builder
+FROM golang:latest AS builder
 
 # install
 RUN go install github.com/pdfcpu/pdfcpu/cmd/pdfcpu@latest