docker-compose.yaml: Default to empty build type

Allow targets to specify a CMake build type but do not set a default
(removed 'Release').  The CXX flags for Release slow the builds down
significantly on Travis-CI (which can hit the time limit for jobs).

Signed-off-by: Noel Eck <noel.eck@intel.com>

docker-compose.yaml

@@ -16,7 +16,7 @@ services:
       - BUILDCORDOVA=${BUILDCORDOVA:-OFF}
       - BUILDSWIGNODE=${BUILDSWIGNODE:-OFF}
       - BUILDEXAMPLES=${BUILDEXAMPLES:-OFF}
-      - CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE:-Release}
+      - CMAKE_BUILD_TYPE
       - IPK=${IPK:-OFF}
       - RPM=${RPM:-OFF}
       - NPM=${NPM:-OFF}