Use golang 1.17.3 in contrib/tkn-image/Dockerfile

- Bump the default GOLANG_VERSION to 1.17.3
  The image was not building with the existing default value, 1.13.* due
  to the use of embeded files

TESTED:
```
docker build --file=contrib/tkn-image/Dockerfile --target=tkn --tag tkn:latest .
```

contrib/tkn-image/Dockerfile

@@ -1,4 +1,4 @@
-ARG GOLANG_VERSION=1.13.8
+ARG GOLANG_VERSION=1.17.13
 ARG DEBIAN_VERSION=10
 
 FROM golang:${GOLANG_VERSION} as builder