Add CI pipeline based on custom Docker image

* Add .gitlab-ci.yml specifying jobs for default builds,
  tests, and debug builds that report compiler warnings.
* Add Dockerfile for building the Docker image used in the
  CI pipeline.
* Add docker/README.md with instruction on how to build and
  upload the image.

.gitlab-ci.yml

+default:
+  image: registry.dune-project.org/lukas.riedel/dune-structures:env
+
+variables:
+  # Add project directory to dunecontrol path
+  DUNE_CONTROL_PATH: /duneci/modules:$CI_PROJECT_DIR
+  CMAKE_FLAGS_DEFAULT:
+    -DDUNE_PYTHON_VIRTUALENV_SETUP=True
+  CMAKE_FLAGS: $CMAKE_FLAGS_DEFAULT
+  # Allow recursive cloning of submodules
+  GIT_SUBMODULE_STRATEGY: recursive
+  # Always void failing 'assert' statement
+  CXXFLAGS_DEFAULT: -DNDEBUG
+  CXXFLAGS: $CXXFLAGS_DEFAULT
+
+workflow:
+  rules:
+    # Trigger pipeline after pushes, starting from web, or scheduling
+    - if: "$CI_PIPELINE_SOURCE == 'push'
+           || $CI_PIPELINE_SOURCE == 'web'
+           || $CI_PIPELINE_SOURCE == 'schedule'"
+
+stages:
+  - build
+  - test
+
+# --- BUILD STAGE ---
+
+# Build release
+build:all:
+  stage: build
+  script:
+    - dunecontrol --current all
+  artifacts:
+    paths:
+      - build-cmake/
+    expire_in: 6h
+
+# Build with debugging and warnings (fail on warnings)
+build:debug:
+  extends: build:all
+  allow_failure: true
+  variables:
+    # Fail on warnings
+    CXXFLAGS: $CXXFLAGS_DEFAULT -Wall -Werror
+    # NOTE: '-Werror' causes QuadMath compile test to fail, so skip it here
+    CMAKE_FLAGS:
+      $CMAKE_FLAGS_DEFAULT
+      -DCMAKE_BUILD_TYPE=Debug
+      -DQuadMath_COMPILES=True
+
+# --- TEST STAGE ---
+
+# Run tests
+test:all:
+  stage: test
+  needs: ["build:all"]
+  script:
+    - dunecontrol --current bexec make test
+  artifacts:
+    paths:
+      - build-cmake/experiments/hansbo_reproduction/
+    expire_in: 1 day

docker/README.md

+# Docker Images
+
+This directory contains source files for building Docker images required to
+build and/or test the `dune-structures` module.
+
+## Building the environment image
+
+The environment image contains all packages for building and testing `dune-structures`.
+It must be built manually using the `build-env.dockerfile`.
+To use it in the GitLab CI/CD, it must be pushed to the container registry.
+Follow these steps:
+
+```bash
+cd docker
+docker build -f build-env.dockerfile -t registry.dune-project.org/lukas.riedel/dune-structures:env .
+docker login registry.dune-project.org
+docker push registry.dune-project.org/lukas.riedel/dune-structures:env
+```

docker/build-env.dockerfile

+FROM registry.dune-project.org/docker/ci/dune:git
+
+ARG DUNECI_PARALLEL=1
+ARG DUNECI_CMAKE_FLAGS="-DDUNE_PYTHON_VIRTUALENV_SETUP=True"
+
+# Base requirements
+USER root
+RUN apt-get update \
+    && apt-get -y upgrade \
+    && apt-get -y install gmsh libyaml-cpp-dev python3-pip python3-venv \
+    && apt-get clean
+USER duneci
+
+# Configure dune-venv
+WORKDIR /duneci/modules
+RUN cmake -DDUNE_PYTHON_VIRTUALENV_SETUP=True dune-common/build-cmake
+
+# alugrid, testtools, pdelab
+RUN duneci-install-module --recursive https://gitlab.dune-project.org/extensions/dune-alugrid.git
+RUN duneci-install-module --recursive https://gitlab.dune-project.org/quality/dune-testtools.git
+RUN duneci-install-module --recursive https://gitlab.dune-project.org/pdelab/dune-pdelab.git
+
+# Build dune-codegen after applying patches
+WORKDIR /duneci/modules
+RUN git clone --recurse-submodules https://gitlab.dune-project.org/extensions/dune-codegen.git
+RUN cd /duneci/modules/dune-codegen && bash patches/apply_patches.sh && cd /duneci/modules/ \
+    && dunecontrol --only=dune-codegen --opts="/duneci/dune.opts" all
+
+# Build dune-blocklab after adding frontend requirements
+WORKDIR /duneci/modules
+RUN git clone --recurse-submodules https://gitlab.dune-project.org/dominic/dune-blocklab.git
+RUN python3 -m pip install -r dune-blocklab/frontend/requirements.txt
+RUN dunecontrol --only=dune-blocklab --opts="/duneci/dune.opts" all