test images: Adds Windows support (part 1)

Adds Windows support to the test/images/image-util.sh script.

A Windows node with Docker installed is required to build Windows images.
The connection URL to it must be set in the REMOTE_DOCKER_URL env variable.
Additionally, the authentication to the remote docker node is done through
certificates, which must be found in ~/.docker.

By default, the REMOTE_DOCKER_URL env variable is set to "" in the Makefile,
and because of it, the image-util.sh script will skip building and pushing
Windows images.

Added GOOS argument to the go build process in order to be able to build
Windows binaries. Additionally, the OS env variable was added to the images
Makefiles (default value is "linux") in order to maintain default behaviour.

Some images require a different Dockerfile for Windows images, since they
have different ways of installing dependencies. Because of this, if a image
needs to be built for Windows, it will first check for a Dockerfile_windows
file instead of the default one. If there isn't one, it means that the
same Dockerfile can be used for both Windows and Linux.

All Windows images will be based on the image
"mcr.microsoft.com/windows/servercore:ltsc2019". There are a couple of features
that are needed from this image, especially powershell.

Added busybox image for Windows. Most Windows images will be based on it, which
will help reduce the command line differences between Linux and Windows, but
not entirely.

Added Windows support for agnhost image.

test/images/apparmor-loader/Dockerfile

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM BASEIMAGE
+ARG BASEIMAGE
+FROM $BASEIMAGE
 
 CROSS_BUILD_COPY qemu-QEMUARCH-static /usr/bin/
 

test/images/apparmor-loader/Makefile

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 SRCS=loader
+OS ?= linux
 ARCH ?= amd64
 TARGET ?= $(CURDIR)
 GOLANG_VERSION ?= latest