Add a build argument for the default shell

[refs #bf31d9c2d568]

Author: jfahrer

myalpine/Dockerfile

@@ -1,9 +1,12 @@
 FROM alpine:latest
 
+ARG MYSHELL=bash
+ENV MYSHELL=$MYSHELL
+
 RUN apk update \
-      && apk add bash \
+      && apk add $MYSHELL \
       && rm -rf /var/cache/apk/*
 
 ENV PS1="\h:\w# " PS2=">> "
 
-CMD bash
+CMD ["sh", "-c", "$MYSHELL"]