manibuilder/alpine: Allow to install additional packages

Change-Id: I2639f611880dd66bd54c18e0934bae5f7b14d04e
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/366
diff --git a/util/manibuilder/Dockerfile.alpine b/util/manibuilder/Dockerfile.alpine
index 408dfc3..32e6436 100644
--- a/util/manibuilder/Dockerfile.alpine
+++ b/util/manibuilder/Dockerfile.alpine
@@ -2,11 +2,13 @@
 
 COPY isrgrootx1.pem /etc/ssl/cert.pem
 
+ARG ADDITIONAL_PACKAGES=
 RUN \
 	adduser -D mani mani && \
 	apk update && \
 	apk add build-base linux-headers git ccache \
-		pciutils-dev libusb-dev libftdi1-dev && \
+		pciutils-dev libusb-dev libftdi1-dev \
+		${ADDITIONAL_PACKAGES} && \
 	if [ ! -h /etc/ssl/cert.pem ]; then \
 		rm -f /etc/ssl/certs/*; \
 		ln -sf /etc/ssl/cert.pem /etc/ssl/certs/ca-certificates.crt; \