buildmaster: Add attr package to get extended attribute support.

The missing {g|s}etfattr commands lead to missing xattr support, falling
back to the unreliable inode based attribute emulation.

Note that the xattr support still depends on the actual filesystem that
is used for the given container. This is unfortunate as it reduces the
reproducibility.

buildmaster/docker/Dockerfile

@@ -1,8 +1,8 @@
 FROM ubuntu:bionic
 
 RUN apt-get update \
-	&& apt-get -y install autoconf automake bison coreutils curl flex gawk gcc \
-		gcc-multilib g++ git libcurl4-openssl-dev make nasm python3 \
+	&& apt-get -y install attr autoconf automake bison coreutils curl flex \
+		gawk gcc gcc-multilib g++ git libcurl4-openssl-dev make nasm python3 \
 		python3-paramiko tar texinfo wget zlib1g-dev \
 	&& apt-get clean