ImageMagick installation in Docker Alpine

Question

So I have this Dockerfile that attempts to install ImageMagick the following way:

FROM ruby:2.4-alpine

...

RUN apk --update add imagemagick

...

The point is that the container doesn't recognizes the file utility (for content-type detection).

Local environment (Mac OSX, installed imagemagick with brew):

> file --version
file-5.25
magic file from /usr/share/file/magic

> convert --version
Version: ImageMagick 6.9.9-5 Q16 x86_64 2017-08-03 
http://www.imagemagick.org
Copyright: © 1999-2017 ImageMagick Studio LLC
License: http://www.imagemagick.org/script/license.php
Features: Cipher DPC Modules
Delegates (built-in): bzlib freetype jng jpeg ltdl lzma png tiff xml zlib

Docker Alpine container (accessed to the shell using docker exec -it CONTAINER_ID):

> file --version
sh: file: not found

> convert --version
Version: ImageMagick 6.9.5-9 Q16 x86_64 2016-10-21         
http://www.imagemagick.org
Copyright: Copyright (C) 1999-2016 ImageMagick Studio LLC
License: http://www.imagemagick.org/script/license.php
Features: Cipher Modules
Delegates (built-in): fontconfig freetype gslib jng jpeg lcms ltdl png ps tiff webp zlib

Also tried installing imagemagick-dev by itself, and both of them combined without any luck (shouldn't make a difference since the first one is a dependency of the latter, I guess).

The question is, how can I install this in the Alpine container? I think I'm missing something but can't figure it out.

By the way, I can't rely on another function other than file for content-type detection since I'm using a framework that explicitly uses this.

Answer 1

The file utility is not part of ImageMagick. It is a standard utility. You can read ore about it at wikipedia:
File (command)

On Alpine Linux, you can install it with apk add --no-cache file:

/ # file /etc/group
/bin/sh: file: not found
/ # apk add --no-cache file
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 libmagic (5.32-r0)
(2/2) Installing file (5.32-r0)
Executing busybox-1.26.2-r5.trigger
OK: 9 MiB in 13 packages
/ # file /etc/group
/etc/group: ASCII text
/ #