The upstream version of dune-alugrid currently wrecks clang 6. I've pushed a possible fix in
extensions/dune-alugrid!49, but until that is merged, switch to a fork which contains that fix.

See merge request docker/ci!20

dune-perftool indirectly depends on libisl and libffi (through some
python dependencies), so make sure we can build them.

Also:
- Update list of images in README
- Fix two small bugs in .gitlab-ci.yml

See merge request docker/ci!19

See merge request docker/ci!18

I had to remove dune-alugrid from the clang-based images because clang
just hangs indefinitely when trying to build that module right now.

See merge request docker/ci!17

ALUGrid causes clang to hang indefinitely during compilation, so there's no use in having it in the
image right now.

This patch puts the virtualenv into `/duneci/modules/dune-python-venv`, which should make it work
in the PDELab + testtools setting.

See merge request docker/ci!16

- Introduce a new build stage "module dependencies" for images that contain everything needed to build a specific module
- Move `dune-pdelab-deps-*` to module dependencies stage
- Add new images `dune-pdelab-{2.6,git}` containing PDELab and dune-testtools

See merge request docker/ci!15

The PDELab images contain both PDELab and dune-testtools to make them usable with
dune-pdelab-systemtesting.

See merge request docker/ci!14

As suggested by Jö, this MR adds some new toolchains that have assertions disabled (`-DNDEBUG`),
as the additional assertion code might hide some bugs.

See merge request docker/ci!13

See merge request docker/ci!12

See merge request docker/ci!11

The distribution package of GMP contains C++ bindings linked against GCC's standard library, causing
linker errors when used with libc++.

See merge request docker/ci!9

- I forgot to update the opts file selection in duneci-standard-test.
- The Docker image was still disabled.
- As there was no additional image file for images with an extra tag,
  downstream images based on those extra tags would use the old image
  instead of the freshly built one. Fixed by adding an extra image file
  that gets skipped during final processing.

See merge request docker/ci!10

That's a stupid bug...

See merge request docker/ci!8

This is a major refactoring of how images are built and how they are used and labelled:

- The base images now export a set of toolchains like `gcc-7-14` or `clang-6-libcpp-17`.
  These are simply defined by sourceable files in `/duneci/toolchains`. There is also a
  default toolchain for each image.
- Based on these toolchains, there is a single opts file called `dune.opts`. This file
  imports the selected toolchain and offers two extension points:
  - Images further down the chain can put files in `/duneci/cmake-flags`. These files
    will be sourced by the opts file after the basic flags have been set up and can
    append / override those flags.
  - Jobs can specify a variable `DUNECI_CMAKE_FLAGS`, which will be appended to the end
    of the flags.
- The core images are now templated on both the base image and the toolchain. Based on
  those parameters, the system now builds a much larger set of core images for different
  combinations of distributions and toolchains. The old images are still available under
  their short names, but the alternative opts files are gone.

See merge request docker/ci!7