usage: go build [-o output] [-i] [build flags] [packages]

Build compiles the packages named by the import paths,
along with their dependencies, but it does not install the results.

go build main
go: import "main": cannot find module providing package main

root@3a67ccb3e0d7:/go/src/mod-test# ls
main.go
root@3a67ccb3e0d7:/go/src/mod-test# cat main.go
package main

import "fmt"

func main() {
	fmt.Println("Hello World.")
}
root@3a67ccb3e0d7:/go/src/mod-test# go build main.go       ## this works
root@3a67ccb3e0d7:/go/src/mod-test# go build main          ## fails
can't load package: package main: cannot find package "main" in any of:
	/usr/local/go/src/main (from $GOROOT)
	/go/src/main (from $GOPATH)

$ go run main.go
build main: cannot find module for path main
$ go run main
go: import "main": cannot find module providing package main

If the arguments to build are a list of .go files, build treats
them as a list of source files specifying a single package.

When compiling a single main package, build writes
the resulting executable to an output file named after
the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe') ...

cd `mktemp -d`
export GOPATH=$(mktemp -d)
mkdir hello
cd hello
go mod init example.com/hello
cat <<EOD > main.go

package main

import "fmt"

func main() {
        fmt.Println("==========")
}
EOD
go build main.go
./main

==========