Update Dockerfile to rust 1.22, latest ImageMagick

Using latest bitflags crate requires a newer version of Rust.

cargo test passes

docker/Dockerfile

@@ -1,10 +1,10 @@
-FROM rust:1.19.0-stretch
+FROM rust:1.22
 
 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 7.0.7-14
+ENV MAGICK_VERSION 7.0.7-15
 
 RUN curl https://www.imagemagick.org/download/ImageMagick-${MAGICK_VERSION}.tar.gz | tar xz \
  && cd ImageMagick-${MAGICK_VERSION} \