switch CI to Alpine v3.6

Alpine v3.6 provides gcc 6.3.0 and clang 4.0.0.

.gitlab-ci.yml

@@ -5,17 +5,16 @@
 # files implicitly passed between jobs or stages, only artifacts and maybe
 # cache (depends on configuration).
+image: alpine:3.6
+
 before_script:
   - case "$CI_BUILD_NAME" in
-      *-clang) export CXX=clang++ EXTRA_PKGS="$EXTRA_PKGS clang@edge2";;
+      *-clang) export CXX=clang++ EXTRA_PKGS="$EXTRA_PKGS clang";;
       *) export CXX=g++;;
     esac
   - export JOBS=$(grep -c processor /proc/cpuinfo)
-  # XXX: Remove after Alpine v3.6 is released and rename tclap-dev@edge to tclap-dev and clang@edge2 to clang.
-  - echo '@edge http://repository.fit.cvut.cz/mirrors/alpine/edge/community' >> /etc/apk/repositories
-  - echo '@edge2 http://repository.fit.cvut.cz/mirrors/alpine/edge/main' >> /etc/apk/repositories
   - apk add --no-cache build-base bash libexecinfo-dev
-        cppunit-dev libxml2-dev tclap-dev@edge $EXTRA_PKGS
+        cppunit-dev libxml2-dev tclap-dev $EXTRA_PKGS
 #==========  Stage build  ==========