docs(ci): document how to build images

.circleci/Dockerfile

@@ -1,4 +1,2 @@
 FROM alpine:3.7
 RUN apk add --no-cache make vim git
-
-# RUN add-apt-repository ppa:jonathonf/vim -y

.circleci/README.md

@@ -0,0 +1,24 @@
+# posva/vim-make
+
+Small docker image with vim, make and git
+
+## Building
+
+```sh
+cd .circleci
+docker rmi vim-make # remove old image
+docker build -t vim-make .
+docker run -it vim-make /bin/date
+docker commit $(docker ps -lq) vim-make
+docker push posva/vim-make:0.3 # replace the tag with a new one
+docker container rm $(docker ps -lq) # remove container
+```
+
+## Cleaning
+
+To remove old images and containers, use `docker images` and `docker ps -a`.
+Then remove them with `docker rmi <image>` and `docker container rm <container>`
+
+## Update `config.yml`
+
+Make sure to update the tag in the `image` section of `.config.yml`