add image

.gitlab-ci.yaml

+stages:
+  - build
+
+build:xpub:base:
+  image: docker:latest
+  stage: build
+  script:
+    - docker version
+    - docker login -u $DOCKERHUB_USERNAME -p $DOCKERHUB_PASSWORD
+    - echo "Ignore warning! Cannot perform an interactive login from a non TTY device"
+    - docker build -t xpub/xpub:base ./images/xpub-base
+    - docker push xpub/xpub:base 
+  only:
+    - master
+  when: manual

README.md

 # `xpub` Deployment Config
 
-This repository contains the templates used by [xpub](https://gitlab.coko.foundation/xpub/xpub) to deploy to Kubernetes.
+This repository contains a base image for [xpub](https://gitlab.coko.foundation/xpub/xpub) and the templates used to deploy to Kubernetes.
+
+## Templates
 
 See [pubsweet/infra](https://gitlab.coko.foundation/pubsweet/infra) and [pubsweet/deployer](https://gitlab.coko.foundation/pubsweet/deployer) for more details about how these templates work.
+
+## Image
+
+The base image for xpub creates a yarn offline mirror from the current `xpub` repository in order to speed up builds there. It can be rebuilt and pushed to docker hub by triggering the relevant job in this repository. The image for `xpub` itself extends this base image.
+

images/xpub-base/Dockerfile

+FROM pubsweet/pubsweet:base
+
+ENV HOME "/home/xpub"
+
+RUN mkdir -p ${HOME}
+
+WORKDIR ${HOME}
+
+# Build yarn offline cache
+
+RUN yarn config set yarn-offline-mirror /npm-packages-offline-cache && \
+  yarn config set prefer-offline true && \
+  yarn config set yarn-offline-mirror-pruning true
+
+RUN git clone https://gitlab.coko.foundation/xpub/xpub.git && \
+    cd xpub && \
+    yarn autoclean --init && yarn && \
+    cd && rm -rf xpub
+