Extend README

# CI images

CI images for use with gitlab.fi.muni.cz

## Provided images

- all images are under the `gitlab.fi.muni.cz:5050/xstill/ci-images` docker

  registry

- there is no `latest` tag, please depend on the particular version to help

  prevent silent changes in build due to image update

The following images are currently available (see the corresponding `Dockerfile` for details):

- `base` -- a minimal environment with `git`, `curl`, `wget` and some other

  utils.

  Has specified locale and timezone and set `DEBIAN_FRONTEND=noninteractive`.

  Used primarily as basis for the other images.

- `haskell-base` -- contains multiple versions of GHC and a reasonably new

  `cabal`.

  The path is set so that no-suffix binaries (e.g. `ghc`, `cabal`) point to the

  newest version and there are also versioned binaries available (for the

  versions see the Dockerfile).

- `texlive` -- a more-or-less complete TeX Live from the time the image was built

  (TeX Live netinstall has poor support for specifying version so it is just the newest).

- `haskell-ib015` -- Haskell image for the IB015 (and IB016) courses.

  Builds on `haskell-base` and `texlive` + has some additional packages.