Apparently the busybox implementation of `base64` will line-wrap long output strings.
This meant that long username+password combinations could produce base64 that
contained spurious "\n" characters, which then led to:
```
2019/05/06 00:47:39 Unable to parse "/kaniko/.docker/config.json": invalid character '\n' in string literal
```
Fixed by just removing the newlines in base64 output. A "better" solution would use a different base64
implementation that avoided line-wrapping in the first place.
