[CI] Remove base image used for CI

Not necessary, since we can just use different images
for our jobs! <3 CircleCI

.circleci/base-image/Dockerfile

@@ -1,22 +0,0 @@
-FROM continuumio/miniconda3:4.5.4
-
-
-RUN apt-get update --yes
-
-RUN apt-get install --yes --no-install-recommends \
-        apt-transport-https \
-        ca-certificates \
-        curl \
-        gnupg2 \
-        software-properties-common
-
-RUN curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
-
-RUN add-apt-repository \
-        "deb [arch=amd64] https://download.docker.com/linux/debian \
-        $(lsb_release -cs) \
-        stable"
-
-RUN apt-get update --yes
-
-RUN apt-get install --yes  docker-ce

.circleci/base-image/README.rst

@@ -1,16 +0,0 @@
-========================
-TLJH CircleCI Base Image
-========================
-
-Miniconda should be installed in the base CircleCI image for our unit tests.
-Docker CE should be installed in the base CircleCI image for our integration tests.
-
-This image contains both, so we do not have to build it every single time.
-
-Updating the image
-==================
-
-1. Make changes to the ``Dockerfile`` & commit it.
-2. Run ``./build.bash`` from this directory. It'll build, tag & push the image with
-   the hash of the last commit that touched this directory.
-3. Use the new image tag in the ``.circleci/config.yml`` file.

.circleci/base-image/build.bash

@@ -1,15 +0,0 @@
-#!/bin/bash
-set -euo pipefail
-
-# Very simple script to build and push images
-# Should be replaced by chartpress or similar at some point
-# Uses google container image builder for simplicity
-IMAGE="jupyterhub/tljh-circleci-base-image"
-TAG=$(git log -n1 --pretty="%h" .)
-IMAGE_SPEC="${IMAGE}:${TAG}"
-
-
-echo "Building and pushing ${IMAGE_SPEC}"
-docker build -t ${IMAGE_SPEC} .
-docker push ${IMAGE_SPEC}
-echo "Built and pushed ${IMAGE_SPEC}"