Set up git workflow to build a maiko docker image with multi-architecture support

On pushes to master: builds images for x86_64, Apple M1, and ARMv7
architectures based on Ubuntu Focal release.  This builds only the
regular lde/ldex and does NOT build the ldeinit, so these are not
suitable for doing a loadup from scratch.

Commit based on https://github.com/stumbo/maiko/tree/gitHubBuild

Dockerfile

@@ -0,0 +1,18 @@
+FROM ubuntu:focal
+ARG BUILD_DATE
+LABEL name="Maiko"
+LABEL description="Virtual machine for Interlisp Medley"
+LABEL url="https://github.com/Interlisp/maiko"
+LABEL build-time=$BUILD_DATE
+
+ARG TARGETPLATFORM
+
+RUN apt-get update && apt-get install -y make clang libx11-dev gcc x11vnc xvfb
+
+COPY --chown=nonroot:nonroot . /app/maiko
+RUN rm -rf /app/maiko/linux*
+
+WORKDIR /app/maiko/bin
+RUN ./makeright x
+
+RUN rm -rf /app/maiko/inc /app/maiko/include /app/maiko/src