Install g++ to build iltorb

Without this, there is a build error:

      CXX(target) Release/obj.target/iltorb/src/common/allocator.o
    make: g++: Command not found

iltorb's Makefile is not in the official GitHub repository, but can be
inspected from an npm install. It looks to be generated by and old
version of gyp¹, and is near identical to this example².

The line `CXX.host ?= g++` is the source of the error. This means it
should be right to install native g++ as it should be used for "host"
purposes only, not to build for a different target architecture.

¹ https://chromium.googlesource.com/external/gyp/+/54d2f6fe6d8a7b9d9786bd1f8540df6b4f46b83f%5E%21/pylib/gyp/generator/make.py
² https://github.com/chentsulin/node-addon-examples/blob/master/pass-parameters/build/Makefile

Dockerfile

@@ -24,6 +24,7 @@ COPY --from=xx / /
 # autoconf, automake: for building VCFtools; may be used by package managers to build from source
 # ca-certificates: for secure HTTPS connections
 # curl: for downloading source files
+# g++: for building iltorb, an indirect dependency of Auspice
 # git: for git clones
 # make: used for building from Makefiles (search for usage); may be used by package managers to build from source
 # pkg-config: for building VCFtools; may be used by package managers to build from source
@@ -35,6 +36,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         clang \
         ca-certificates \
         curl \
+        g++ \
         git \
         make \
         pkg-config \