miqt-docker: Support additional ldflags when using windows-build, minify-build

In miqt-docker, normally you can pass an entire Go build command (miqt-docker ENV go build ...) For the predefined windows-build and minify-build commands, additional arguments do go through (miqt-docker ENV -windows-build FOO expands to miqt-docker ENV go build special_windows_arguments FOO) But because special_windows_arguments includes an -ldflags argument, it's not possible to specify additional ldflags arguments. This stops you doing something like `-ldflags=-x main.MY_APP_VERSION=$(git_get_tag)` for release builds. For the special predefined comamnds, miqt-docker should detect additional ldflags arguments and merge them properly into the main, internal ldflag argument.