ci-base: Document building and pushing a new image

ci/Dockerfile

@@ -1,7 +1,8 @@
 FROM elixir:1.10.4
 
+# Single RUN statement, otherwise intermediate images are created
+# https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run
 RUN apt-get update &&\
-  apt-get install -y libmagic-dev cmake libimage-exiftool-perl ffmpeg &&\
+	apt-get install -y libmagic-dev cmake libimage-exiftool-perl ffmpeg &&\
 	mix local.hex --force &&\
 	mix local.rebar --force
-

ci/README

@@ -0,0 +1,12 @@
+## Dependencies
+
+Assuming an AMD64 Alpine system, you're going to need the following packages
+- `qemu qemu-openrc qemu-arm qemu-aarch64` for binfmt
+- `docker-cli-buildx` for building the images
+
+## Setting up
+
+```
+docker login git.pleroma.social:5050
+doas rc-service qemu-binfmt start
+```