BUILDING.md: add netgo for static build

When compiling containerd binaries statically, linker rightfully
complains:

```
+ make BUILDTAGS=static_build 'EXTRA_FLAGS=-buildmode pie'
'EXTRA_LDFLAGS=-extldflags "-fno-PIC -static"'
🇩 bin/ctr
/tmp/go-link-343047789/000000.o: In function
`_cgo_b0c710f30cfd_C2func_getaddrinfo':
/tmp/go-build/net/_obj/cgo-gcc-prolog:46: warning: Using 'getaddrinfo'
in statically linked applications requires at runtime the shared
libraries from the glibc version used for linking
```

The same error appears for ctr, containerd, and containerd-stress
binaries.

The fix is to use Go's own DNS resolver functions, rather than
glibc's getaddrinfo() -- this option is turned on by `netgo` build
tag.

See https://golang.org/pkg/net/ (look for "Name Resolution") for
more details.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

BUILDING.md

@@ -84,7 +84,7 @@ You can build static binaries by providing a few variables to `make`:
 ```sudo
 make EXTRA_FLAGS="-buildmode pie" \
 	EXTRA_LDFLAGS='-extldflags "-fno-PIC -static"' \
-	BUILDTAGS="static_build"
+	BUILDTAGS="static_build netgo"
 ```
 
 > *Note*: