Add additional Docker archs

Fixes: #87

Signed-off-by: Ben Kochie <superq@gmail.com>

Author: SuperQ

Makefile

@@ -12,7 +12,7 @@
 # limitations under the License.
 
 # Needs to be defined before including Makefile.common to auto-generate targets
-DOCKER_ARCHS ?= amd64
+DOCKER_ARCHS ?= amd64 armv7 arm64 ppc64le s390x
 DOCKER_REPO  ?= prometheuscommunity
 
 include Makefile.common