Fix the fully static image and the build image for the C++ sources

docker/static: the C++ unit tests link the static libstdc++, whose error
paths need an unwinder that --rtlib=compiler-rt does not provide; link
llvm-libunwind. The programs themselves still need no C++ runtime.

docker/build: build in the default bundled mode with CMake instead of
against libev/udns (Alpine's Mbed TLS 4 is not supported by system
mode).

docker/build/builder.Dockerfile

@@ -1,8 +1,7 @@
-# Alpine with China mirror
+# Development image for building shadowsocks-c. The default bundled mode
+# builds every dependency from third_party/dist, so only a toolchain is
+# needed (Alpine's Mbed TLS 4 is not supported by system mode).
 FROM alpine
-MAINTAINER wener <wenermail@gmail.com>
 
-# Better for cache and dev
 RUN apk add --no-cache --virtual .build-deps \
-    alpine-sdk cmake \
-    linux-headers libev-dev libsodium-dev mbedtls-static mbedtls-dev pcre2-dev udns-dev
+    build-base cmake ninja python3 linux-headers

docker/build/dockerbuild.sh

@@ -2,4 +2,7 @@
 set -e
 set -o xtrace
 
-cmake -DBUILD_STATIC=OFF . && make && make install
+cmake -S . -B build -G Ninja
+cmake --build build
+ctest --test-dir build -L 'unit|vendor' --output-on-failure
+cmake --install build