Add missing libs for debian

- `libbrotli-dev` for `mod_brotli`
- `libjansson` and `libcurl` for `mod_md`

2.4/Dockerfile

@@ -37,6 +37,8 @@ ENV APACHE_DIST_URLS \
 # see https://httpd.apache.org/docs/2.4/install.html#requirements
 RUN set -eux; \
 	\
+	# libbrotli is only in backports: https://packages.debian.org/stretch-backports/libbrotli-dev
+	echo 'deb http://deb.debian.org/debian stretch-backports main' > /etc/apt/sources.list.d/stretch-backports.list; \
 	# mod_http2 mod_lua mod_proxy_html mod_xml2enc
 	# https://anonscm.debian.org/cgit/pkg-apache/apache2.git/tree/debian/control?id=adb6f181257af28ee67af15fc49d2699a0080d4c
 	savedAptMark="$(apt-mark showmanual)"; \
@@ -48,6 +50,9 @@ RUN set -eux; \
 		dpkg-dev \
 		gcc \
 		gnupg \
+		libbrotli-dev \
+		libcurl4-openssl-dev \
+		libjansson-dev \
 		liblua5.2-dev \
 		libnghttp2-dev \
 		libpcre3-dev \