I was running a build of my site when I hit the following error: “Can’t install RMagick 2.16.0. Can’t find MagickWand.h.”

I had the following logs (I’ve abbreviated them for space):

Build started.
Pulling image jekyll/jekyll:3.4
Creating image started.
Sending build context to Docker daemon  2.048kB

Step 1/10 : FROM jekyll/jekyll:3.4
 ---> 131e7647f128
Step 2/10 : RUN apk add --no-cache imagemagick
 ---> Running in 459d10b95ee4
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
.
.
.
(20/20) Installing imagemagick (7.0.5.10-r0)
Step 3/10 : RUN apk add --no-cache imagemagick-dev
 ---> Running in edee017fe6d3
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
(1/2) Installing imagemagick-c++ (7.0.5.10-r0)
(2/2) Installing imagemagick-dev (7.0.5.10-r0)
.
.
.
Step 7/10 : RUN gem install rmagick
 ---> Running in a2c976ea58ce
Building native extensions.  This could take a while...
ERROR:  Error installing rmagick:
	ERROR: Failed to build gem native extension.

    current directory: /usr/local/bundle/gems/rmagick-2.16.0/ext/RMagick
/usr/local/bin/ruby -r ./siteconf20170831-7-nke7aj.rb extconf.rb
checking for gcc... yes
checking for Magick-config... no
checking for pkg-config... yes
checking for outdated ImageMagick version (<= 6.4.9)... no
checking for presence of MagickWand API (ImageMagick version >= 6.9.0)... no
checking for Ruby version >= 1.8.5... yes
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... no

Can't install RMagick 2.16.0. Can't find MagickWand.h.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.
.
.
.
The command '/bin/sh -c gem install rmagick' returned a non-zero code: 1
Build failed !!!.

After quite a bit of Google it turns out that RMagick 2.16.0 is not compatible with the newest version of ImageMagick (Version 7), but it looks like its only on older versions of Ruby.

It looks like my only way out of this is to install an older version of ImageMagick - specifically version 6, but on Alpine Linux that doesn’t seem to be as straight forward as other versions of linux. Then I came across this superuser answer by Vlad Frolov. Turns out I had to add the repo for the older version of Alpine using the following command:

echo 'http://dl-cdn.alpinelinux.org/alpine/v3.5/main' >> /etc/apk/repositories

Then I could specifically request the older version of ImageMagick

apk add --no-cache imagemagick=6.9.6.8-r1

Then my build proceeded as before. Problem (temporarily I hope) solved.