build: Statically link the toolbox binary

The Go implementation of Toolbox uses in few places code written in C
(not directly in the Toolbox code but in some library). This causes
Toolbox to be dynamically linked to the systems C library (libc). Under
other circumstances this would not be a problem but Toolbox itself is
being used as the entry-point of toolbox containers. The most widely
used implementation of libc is glibc. Its versions seem to be
backwards-compatible but not forward-compatible, making Toolbox unusable
in older distros that ship a previous version of glibc.

One of the solutions is to statically link libc (resp. glibc) which is
what this commit does. The size of the final binary seems to not be
affected.

CGO_ENABLED=0 forces the build not to use cgo making it effectively
statically linked and '-tags osusergo' forces the use of purely Go
implementation of os/user.

https://golang.org/pkg/os/user/
https://golang.org/cmd/cgo/

#531

src/go-build-wrapper

@@ -27,5 +27,5 @@ if ! cd "$1"; then
     exit 1
 fi
 
-go build -trimpath -ldflags "-X github.com/containers/toolbox/pkg/version.currentVersion=$3" -o "$2"
+CGO_ENABLED=0 go build -tags osusergo -trimpath -ldflags "-X github.com/containers/toolbox/pkg/version.currentVersion=$3" -o "$2"
 exit "$?"