Set Rust docker image version to "latest"

ImageMagick is still set to 7.0.x for now.

docker/Dockerfile

@@ -1,18 +1,18 @@
-FROM rust:1.22
+FROM rust:latest
 
 RUN apt-get update \
  && apt-get -y install curl build-essential clang pkg-config libjpeg-turbo-progs libpng-dev \
  && rm -rfv /var/lib/apt/lists/*
 
-ENV MAX_MAGICK_VERSION 7.0
+ENV MAGICK_VERSION 7.0
 
 RUN curl https://www.imagemagick.org/download/ImageMagick.tar.gz | tar xz \
- && cd ImageMagick-${MAX_MAGICK_VERSION}* \
+ && cd ImageMagick-${MAGICK_VERSION}* \
  && ./configure --with-magick-plus-plus=no --with-perl=no \
  && make \
  && make install \
  && cd .. \
- && rm -r ImageMagick-${MAX_MAGICK_VERSION}*
+ && rm -r ImageMagick-${MAGICK_VERSION}*
 
 RUN adduser --disabled-password --gecos '' magick-rust