merge full and slim dockerfiles

Fixes #232

Dockerfile.alpine

@@ -28,8 +28,13 @@ RUN set -xe && \
 
 FROM alpine:3.12
 
+# Set to "true" to install the optional git and rsync dependencies
+ARG INSTALL_OPTIONAL_PACKAGES=false
+
 RUN apk add --update --no-cache ca-certificates tzdata mailcap
 
+RUN if [ "${INSTALL_OPTIONAL_PACKAGES}" = "true" ]; then apk add --update --no-cache rsync git; fi
+
 # set up nsswitch.conf for Go's "netgo" implementation
 # https://github.com/gliderlabs/docker-alpine/issues/367#issuecomment-424546457
 RUN test ! -e /etc/nsswitch.conf && echo 'hosts: files dns' > /etc/nsswitch.conf