Dockerfile: Use Go 1.20 by default (dutchcoders#550)

Use Go 1.20 by default in Dockerfile

Updating the default Go version used in the Dockerfile, to the latest
one supported by the project at the moment.

Also, Go `1.17` support was removed in
dutchcoders@9f1fe62

Dockerfile

@@ -1,5 +1,5 @@
-# Default to Go 1.17
-ARG GO_VERSION=1.17
+# Default to Go 1.20
+ARG GO_VERSION=1.20
 FROM golang:${GO_VERSION}-alpine as build
 
 # Necessary to run 'go get' and to compile the linked binary