lock ImageMagick to 7.0 in Dockerfile

Specifying a concrete version of IM required updating the Dockerfile
every time a new release is pushed, which happens pretty often.

docker/Dockerfile

@@ -4,15 +4,15 @@ 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 MAGICK_VERSION 7.0.7-15
+ENV MAX_MAGICK_VERSION 7.0
 
-RUN curl https://www.imagemagick.org/download/ImageMagick-${MAGICK_VERSION}.tar.gz | tar xz \
- && cd ImageMagick-${MAGICK_VERSION} \
+RUN curl https://www.imagemagick.org/download/ImageMagick.tar.gz | tar xz \
+ && cd ImageMagick-${MAX_MAGICK_VERSION}* \
  && ./configure --with-magick-plus-plus=no --with-perl=no \
  && make \
  && make install \
  && cd .. \
- && rm -r ImageMagick-${MAGICK_VERSION}
+ && rm -r ImageMagick-${MAX_MAGICK_VERSION}*
 
 RUN adduser --disabled-password --gecos '' magick-rust