I came across the same blogpost and the docker image based on the blogpost. I tried crosscompiling Filebeat for Solaris on SPARC the same way, but I was not successful compiling it. I am glad to share my findings with you. I had the same problems like you.
The package plugin of Golang stdlib is only supported on Linux. The build tag on the file is correctly set to only build it when GOOS is linux. You need to pass GOOS=solaris so the appropiate source files are selected. However, I was not able to pass this variable to the compiler, so couldn't compile it. I also tried GOOS=solaris go build --compiler gccgo and so on, but so far no luck.
My next move would be to try to compile Filebeat on a solaris/sparc machine, if I had access to one. 