Fix breaking docker build

The Dockerfile.apache has been updated recently, but Dockerfile.fpm not. Due to this, the docker build is failing with the following error:
```
configure: error: unrecognized options: --enable-freetype
The command '/bin/sh -c apt-get update  && apt-get install -y --no-install-recommends apt-utils  && apt-get install -y --no-install-recommends git gosu       optipng pngquant jpegoptim gifsicle libpq-dev libsqlite3-dev locales zip unzip libzip-dev libcurl4-openssl-dev       libfreetype6 libicu-dev libjpeg62-turbo libpng16-16 libxpm4 libwebp6 libmagickwand-6.q16-6       libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libmagickwand-dev mariadb-client && sed -i '/en_US/s/^#//g' /etc/locale.gen  && locale-gen && update-locale  && docker-php-source extract  && docker-php-ext-configure gd       --enable-freetype       --with-jpeg-dir=/usr/lib/x86_64-linux-gnu/       --with-xpm-dir=/usr/lib/x86_64-linux-gnu/       --with-webp-dir=/usr/lib/x86_64-linux-gnu/  && docker-php-ext-install pdo_mysql pdo_pgsql pdo_sqlite pcntl gd exif bcmath intl zip curl  && pecl install imagick  && docker-php-ext-enable imagick pcntl imagick gd exif zip curl  && curl -LsS https://getcomposer.org/download/${COMPOSER_VERSION}/composer.phar -o /usr/bin/composer  && echo "${COMPOSER_CHECKSUM}  /usr/bin/composer" | sha256sum -c -  && chmod 755 /usr/bin/composer  && apt-get autoremove --purge -y        libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libvpx-dev libmagickwand-dev  && rm -rf /var/cache/apt  && docker-php-source delete' returned a non-zero code: 1
```

This change updates Dockerfile.fpm to equal the Dockerfile.apache variant. (Except the fpm or apache specific settings)

contrib/docker/Dockerfile.fpm

@@ -1,7 +1,5 @@
-FROM php:7.3-fpm-buster
-
-ARG COMPOSER_VERSION="1.8.5"
-ARG COMPOSER_CHECKSUM="4e4c1cd74b54a26618699f3190e6f5fc63bb308b13fa660f71f2a2df047c0e17"
+ARG COMPOSER_VERSION="1.9.1"
+ARG COMPOSER_CHECKSUM="1f210b9037fcf82670d75892dfc44400f13fe9ada7af9e787f93e50e3b764111"
 
 RUN apt-get update \
  && apt-get install -y --no-install-recommends apt-utils \
@@ -13,13 +11,12 @@ RUN apt-get update \
  && locale-gen && update-locale \
  && docker-php-source extract \
  && docker-php-ext-configure gd \
-      --enable-freetype \
-      --with-jpeg-dir=/usr/lib/x86_64-linux-gnu/ \
-      --with-xpm-dir=/usr/lib/x86_64-linux-gnu/ \
-      --with-webp-dir=/usr/lib/x86_64-linux-gnu/ \
+      --with-freetype \
+      --with-jpeg \
+      --with-webp \
+      --with-xpm \
  && docker-php-ext-install pdo_mysql pdo_pgsql pdo_sqlite pcntl gd exif bcmath intl zip curl \
- && pecl install imagick \
- && docker-php-ext-enable imagick pcntl imagick gd exif zip curl \
+ && docker-php-ext-enable pcntl gd exif zip curl \
  && curl -LsS https://getcomposer.org/download/${COMPOSER_VERSION}/composer.phar -o /usr/bin/composer \
  && echo "${COMPOSER_CHECKSUM}  /usr/bin/composer" | sha256sum -c - \
  && chmod 755 /usr/bin/composer \
@@ -33,7 +30,7 @@ ENV PATH="~/.composer/vendor/bin:./vendor/bin:${PATH}"
 COPY . /var/www/
 
 WORKDIR /var/www/
-RUN mkdir public.ext && cp -r storage storage.skel \
+RUN cp -r storage storage.skel \
  && cp contrib/docker/php.ini /usr/local/etc/php/conf.d/pixelfed.ini \
  && composer install --prefer-dist --no-interaction \
  && rm -rf html && ln -s public html