[TravisCI] Refactor as many CI default options as possible so that

the Docker and "TravisCI macOS" builds share most of the same defaults
by sourcing the `ci_defaults.sh` file.

contrib/ci/Dockerfiles/z3_build.Dockerfile

@@ -2,34 +2,33 @@ ARG DOCKER_IMAGE_BASE
 FROM ${DOCKER_IMAGE_BASE}
 
 
-# Specify defaults. This can be changed when invoking
+# Build arguments. This can be changed when invoking
 # `docker build`.
-ARG ASAN_BUILD=0
-ARG BUILD_DOCS=0
-ARG CC=gcc
-ARG CXX=g++
-ARG DOTNET_BINDINGS=1
-ARG JAVA_BINDINGS=1
-ARG NO_SUPPRESS_OUTPUT=0
-ARG PYTHON_BINDINGS=1
-ARG PYTHON_EXECUTABLE=/usr/bin/python2.7
-ARG RUN_SYSTEM_TESTS=1
-ARG RUN_UNIT_TESTS=1
-ARG TARGET_ARCH=x86_64
-ARG TEST_INSTALL=1
-ARG UBSAN_BUILD=0
-ARG USE_LIBGMP=0
-ARG USE_LTO=0
-ARG USE_OPENMP=1
+ARG ASAN_BUILD
+ARG BUILD_DOCS
+ARG CC
+ARG CXX
+ARG DOTNET_BINDINGS
+ARG JAVA_BINDINGS
+ARG NO_SUPPRESS_OUTPUT
+ARG PYTHON_BINDINGS
+ARG PYTHON_EXECUTABLE
+ARG RUN_SYSTEM_TESTS
+ARG RUN_UNIT_TESTS
+ARG TARGET_ARCH
+ARG TEST_INSTALL
+ARG UBSAN_BUILD
+ARG USE_LIBGMP
+ARG USE_LTO
+ARG USE_OPENMP
 ARG Z3_SRC_DIR=/home/user/z3_src
-ARG Z3_BUILD_TYPE=RelWithDebInfo
-ARG Z3_CMAKE_GENERATOR=Ninja
-ARG Z3_INSTALL_PREFIX=/usr
-ARG Z3_STATIC_BUILD=0
-# Blank default indicates use latest.
+ARG Z3_BUILD_TYPE
+ARG Z3_CMAKE_GENERATOR
+ARG Z3_INSTALL_PREFIX
+ARG Z3_STATIC_BUILD
 ARG Z3_SYSTEM_TEST_GIT_REVISION
-ARG Z3_WARNINGS_AS_ERRORS=SERIOUS_ONLY
-ARG Z3_VERBOSE_BUILD_OUTPUT=0
+ARG Z3_WARNINGS_AS_ERRORS
+ARG Z3_VERBOSE_BUILD_OUTPUT
 
 ENV \
   ASAN_BUILD=${ASAN_BUILD} \
@@ -74,6 +73,7 @@ ADD *.txt *.md RELEASE_NOTES ${Z3_SRC_DIR}/
 
 ADD \
   /contrib/ci/scripts/build_z3_cmake.sh \
+  /contrib/ci/scripts/ci_defaults.sh \
   /contrib/ci/scripts/set_compiler_flags.sh \
   /contrib/ci/scripts/set_generator_args.sh \
   ${Z3_SRC_DIR}/contrib/ci/scripts/