Add base image for mksh

This is a minimal standalone statically linked shell for use
for now in converting images to containers.

Plan to phase it out and replace with actual programs later.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/base/mksh/Dockerfile

@@ -0,0 +1,15 @@
+# Tag: 9e9c6b252d2e4ec03da23c8f48f54fce7ddee8d3
+FROM mobylinux/alpine-build-c@sha256:68737dcc6a1081f07aace1e82aefe90df399a25ae3a025664c4dbdccf76bbd97
+
+ENV VERSION=mksh-R54
+
+RUN curl -O -sSL https://github.com/MirBSD/mksh/archive/$VERSION.tar.gz
+RUN zcat $VERSION.tar.gz | tar xvf -
+
+WORKDIR mksh-$VERSION
+
+ENV LDFLAGS=-static
+
+RUN sh ./Build.sh
+RUN strip mksh
+RUN install -c -s -o root -g bin -m 555 mksh /bin/mksh