Add multi-architecture Docker image build

* Adds a shell script with functions to install multi-architecture docker
  support, as well as build, deploy, and test the shellcheck docker images for
  the same set of architectures for which binaries were already built and
  deployed as tarballs.
* Hooks up the multi-architecture docker build, deploy, and test to the existing
  Travis CI/CD pipeline. It is organized as a separate stage which only runs if
  all previous steps in the already existing test stage succeed.

Dockerfile.multi-arch

@@ -0,0 +1,26 @@
+# Alpine image
+FROM alpine:latest AS alpine
+LABEL maintainer="Vidar Holen <vidar@vidarholen.net>"
+ARG tag
+
+# Put the right binary for each architecture into place for the
+# multi-architecture docker image.
+RUN set -x; \
+  arch="$(uname -m)"; \
+  echo "arch is $arch"; \
+  if [ "${arch}" = 'armv7l' ]; then \
+    arch='armv6hf'; \
+  fi; \
+  url_base='https://shellcheck.storage.googleapis.com/'; \
+  tar_file="shellcheck-${tag}.linux.${arch}.tar.xz"; \
+  wget "${url_base}${tar_file}" -O - | tar xJf -; \
+  mv "shellcheck-${tag}/shellcheck" /bin/; \
+  rm -rf "shellcheck-${tag}"; \
+  ls -laF /bin/shellcheck
+
+# ShellCheck image
+FROM scratch
+LABEL maintainer="Vidar Holen <vidar@vidarholen.net>"
+WORKDIR /mnt
+COPY --from=alpine /bin/shellcheck /bin/
+ENTRYPOINT ["/bin/shellcheck"]