Add webp support

See https://make.wordpress.org/core/2021/06/07/wordpress-5-8-adds-webp-support/

Author: tianon

Dockerfile.template

@@ -49,6 +49,7 @@ RUN set -ex; \
 		imagemagick-dev \
 		libjpeg-turbo-dev \
 		libpng-dev \
+		libwebp-dev \
 		libzip-dev \
 	; \
 {{ ) else ( -}}
@@ -60,6 +61,7 @@ RUN set -ex; \
 		libjpeg-dev \
 		libmagickwand-dev \
 		libpng-dev \
+		libwebp-dev \
 		libzip-dev \
 	; \
 {{ ) end -}}
@@ -69,9 +71,11 @@ RUN set -ex; \
 		--with-freetype-dir=/usr \
 		--with-jpeg-dir=/usr \
 		--with-png-dir=/usr \
+		--with-webp-dir=/usr \
 {{ ) else ( -}}
 		--with-freetype \
 		--with-jpeg \
+		--with-webp \
 {{ ) end -}}
 	; \
 	docker-php-ext-install -j "$(nproc)" \