Add imagemagick-heic to the Alpine variants

[LeoFidjeland]

Alpine packages ImageMagick's coders as separate subpackages, and the base imagemagick
package ships none of them. So on the Alpine images Imagick::queryFormats('AVIF') is
empty, WP_Image_Editor_Imagick::supports_mime_type('image/avif') returns false, and
_wp_image_editor_choose() falls back to WP_Image_Editor_GD.

That fallback is lossy. GD's imageavif() discards the ICC profile and tags output as
sRGB, so uploads carrying Adobe RGB or Display P3 profiles get reinterpreted as sRGB and
render visibly desaturated. Nothing errors — the AVIF files are produced successfully,
just without their colour information — which makes this easy to run into without
realising. The Debian variants are unaffected, because there libmagickcore links libheif
directly.

This is the Alpine counterpart to #996 (fixed for Debian in #997). The difference is that #996 was decode-only
support failing loudly at encode time (no encode delegate for this image format 'AVIF'),
whereas here Imagick has no AVIF support at all, so it fails the editor-capability check
and WordPress quietly uses a different editor.

Before

$ docker run --rm wordpress:fpm-alpine php -r 'var_dump(Imagick::queryFormats("AVIF"));'
array(0) {
}

$ docker run --rm wordpress:fpm-alpine sh -c 'ls /usr/lib/ImageMagick-*/modules-*/coders/ | grep -c heic'
0

After

Built from latest/php8.3/fpm-alpine with this change:

$ docker run --rm wp-heic-test php -r 'var_dump(Imagick::queryFormats("AVIF"));'
array(1) {
  [0]=>
  string(4) "AVIF"
}

AVIF encodes, and the profile survives a JPEG → AVIF conversion:

src icc: 560 bytes (Adobe RGB (1998))
out icc: 560 bytes

Measured mean saturation on a real Adobe RGB photograph: source JPEG 44.25%, Imagick AVIF
44.41%. The GD output the images currently produce is visibly lower.

On size

The image grows 3 MB, from 310.4 MB to 313.4 MB.

AVIF encoding needs no new heavy dependency: libavif, aom-libs and libdav1d are
already in the image for GD's --with-avif, and imagemagick-heic brings libheif-aom
with it. The added weight is libheif itself (1.6 MB), libde265 (0.6 MB) and a few small
plugin shims; the ImageMagick coder module is 66 KiB.

I initially also added libheif-x265 for HEVC parity with the Debian variants after #997,
but dropped it - it is 6.8 MB on its own, more than twice the cost of this fix, and HEIC
encoding is not what breaks here. Happy to add it in a follow-up if that parity is wanted.

Dockerfile.template was edited and ./apply-templates.sh run; the 8 generated
fpm-alpine Dockerfiles are the only other files changed.