build: re-create the builder docker image when Dockerfile changes

It takes longer but works all the time. Uploading an image to a docker
repository to speed-up the build requires an additional manual step
and access to the docker repository.

A possible optimization (would be a few minutes faster) could be
implemented by storing docker layers, similar to what is done for the
Dockerfile used when testing.

molecule/builder/Dockerfile

@@ -1,4 +1,5 @@
-FROM ubuntu:14.04
+# ubuntu:14.04 as of 2017-12-15
+FROM ubuntu@sha256:084989eb923bd86dbf7e706d464cf3587274a826b484f75b69468c19f8ae354c
 
 RUN apt-get -y update && apt-get install -y python rsync sudo bash \
             devscripts git aptitude ipython libssl-dev ntp \

molecule/builder/create.yml

@@ -8,13 +8,17 @@
     molecule_ephemeral_directory: "{{ lookup('env', 'MOLECULE_EPHEMERAL_DIRECTORY') }}"
     molecule_scenario_directory: "{{ lookup('env', 'MOLECULE_SCENARIO_DIRECTORY') }}"
     molecule_yml: "{{ lookup('file', molecule_file) | from_yaml }}"
-    docker_hash: "{{ lookup('file', '../docker_hashes.yml') | from_yaml }}"
   tasks:
+    - name: Create builder image
+      docker_image:
+        name: builder
+        path: .
+        force: yes
     - name: Create molecule instance(s)
       docker_container:
         name: "{{ item.name }}"
         hostname: "{{ item.name }}"
-        image: "{{ item.image }}@{{ docker_hash[item.hash_name] }}"
+        image: builder
         state: started
         recreate: True
         command: "tail -f /dev/null"

molecule/builder/molecule.yml

@@ -5,8 +5,6 @@ lint:
   name: yamllint
 platforms:
   - name: build
-    image: quay.io/freedomofpress/ubuntu:trusty
-    hash_name: ubuntu
     groups:
       - build
 provisioner: