For linux arm64 and amd64, as per #586.
I moved the builder image into the same dockerfile, and bumped the Go
version on it. It didn't seem like the builder dockerfile worked with
the latest code anyway (the go modules require go 1.15 and higher).
This requires a recent enough docker install with buildx, as well as
an arm64 builder.
BASEIMAGE is changed to not specify an arch, so that the image will
build on its native arch in buildx.
Example image is on docker hub as:
kelvie/node-problem-detector:v0.8.10-5-gb0fa610