.gitlab-ci.yml: use pre-installed images for gnutls

This avoids the need to install packages during check, and
reduces failures due to infrastructure.
Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>

.gitlab-ci.yml

-image: centos:7
+variables:
+  BUILD_IMAGES_PROJECT: gnutls/build-images
+  FEDORA_BUILD: buildenv-f26
+  FEDORA_X86_BUILD: buildenv-f26-x86
+  GET_SOURCES_ATTEMPTS: "3"
 
+# remove any pre-installed headers from nettle
 before_script:
- - yum install -y git make autoconf libtool automake gettext-devel glibc-devel gcc valgrind libasan-static libgcc.i686 glibc-devel.i686
+ - yum remove -y nettle-devel
 
 # See http://doc.gitlab.com/ce/ci/yaml/ for documentation.
 build/x86-64:
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
-  - yum install -y gmp-devel
   - ./.bootstrap &&
     ./configure --enable-fat --disable-documentation && make -j4 &&
     make check -j4
@@ -15,8 +20,8 @@ build/x86-64:
   except:
   - tags
 build/x86:
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_X86_BUILD
   script:
-  - yum install -y gmp-devel.i686
   - ./.bootstrap &&
     PKG_CONFIG_PATH="/usr/lib/pkgconfig/" CFLAGS="-O2 -g -m32" LDFLAGS="-m32" ./configure --build=i686-redhat-linux --enable-fat --disable-documentation && make -j4 &&
     make check -j4
@@ -38,6 +43,7 @@ build/aarch64:
   except:
   - tags
 build/mini-gmp:
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
   - ./.bootstrap &&
     ./configure --disable-documentation --enable-mini-gmp && make -j4 &&
@@ -47,9 +53,8 @@ build/mini-gmp:
   except:
   - tags
 build/ubsan:
-  image: fedora:26
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
-  - yum install -y gmp-devel libubsan-static libubsan
   - ./.bootstrap && 
     CXXFLAGS="-fsanitize=undefined -fno-sanitize-recover -g -O2" CFLAGS="-fsanitize=undefined -fno-sanitize-recover -g -O2" ./configure 
     --disable-documentation && make -j4 && make check -j4
@@ -58,9 +63,8 @@ build/ubsan:
   except:
   - tags
 build/asan:
-  image: fedora:26
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
-  - yum install -y gmp-devel
   - ./.bootstrap && 
   - CXXFLAGS="-fsanitize=address -g -O2" CFLAGS="-fsanitize=address -g -O2" ./configure --disable-documentation &&
     make -j4 && make check -j4
@@ -69,9 +73,8 @@ build/asan:
   except:
   - tags
 build/static-analyzers:
-  image: fedora:26
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
-  - yum install -y gmp-devel clang compiler-rt cppcheck clang-analyzer
   - ./.bootstrap
   - scan-build ./configure --disable-documentation
   - scan-build --status-bugs -o scan-build-lib make -j$(nproc)
@@ -84,4 +87,3 @@ build/static-analyzers:
     when: on_failure
     paths:
       - scan-build-lib/*
-