images: Allow base image substitution for Drydock

Currently, Drydock images are built using the Ubuntu:16.04 base image.
This change allows users to specify different base images using the
UBUNTU_BASE_IMAGE build argument.

Change-Id: I9ddaa89eb5262571703a3dbf7ebb6deed1505842

Makefile

@@ -18,6 +18,7 @@ IMAGE_NAME      ?= drydock
 IMAGE_PREFIX    	?= airshipit
 IMAGE_TAG       	?= dev
 HELM            	:= $(shell realpath $(BUILD_DIR))/helm
+UBUNTU_BASE_IMAGE	?=
 PROXY           	?= http://proxy.foo.com:8000
 NO_PROXY        	?= localhost,127.0.0.1,.svc.cluster.local
 USE_PROXY       	?= false

images/drydock/Dockerfile

@@ -11,7 +11,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-FROM ubuntu:16.04
+
+ARG FROM=ubuntu:16.04
+FROM ${FROM}
 
 LABEL org.opencontainers.image.authors='airship-discuss@lists.airshipit.org, irc://#airshipit@freenode'
 LABEL org.opencontainers.image.url='https://airshipit.org'

tools/drydock_image_build.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 set -x
 
+UBUNTU_BASE_IMAGE=${UBUNTU_BASE_IMAGE:-""}
 UBUNTU_REPO=${UBUNTU_REPO:-""}
 TRUSTED_UBUNTU_REPO=${TRUSTED_UBUNTU_REPO:-"no"}
 ALLOW_UNATHENTICATED=${ALLOW_UNAUTHENTICATED:-"false"}
@@ -9,6 +10,11 @@ PIP_TRUSTED_HOST=${PIP_TRUSTED_HOST:-""}
 
 ADDL_BUILD_ARGS=""
 
+if [[ ! -z "${UBUNTU_BASE_IMAGE}" ]]
+then
+  ADDL_BUILD_ARGS="${ADDL_BUILD_ARGS} --build-arg FROM=${UBUNTU_BASE_IMAGE}"
+fi
+
 if [[ ! -z "${UBUNTU_REPO}" ]]
 then
   ADDL_BUILD_ARGS="${ADDL_BUILD_ARGS} --build-arg UBUNTU_REPO=${UBUNTU_REPO}"