In order to compile a fully static binary when using Cgo you'll need to link in a C library like musl.
I find it convenient to have a Docker image ready for building these artifacts.
FROM golang
RUN wget https://www.musl-libc.org/releases/musl-1.2.0.tar.gz && \
tar -zf musl-1.2.0.tar.gz && \
cd musl-1.2.0 && \
./configure --enable-static --disable-shared && \