Fix dockerfile (#5059)

substrate/.maintain/Dockerfile

 # Note: We don't use Alpine and its packaged Rust/Cargo because they're too often out of date,
 # preventing them from being used to build Substrate/Polkadot.
 
-FROM phusion/baseimage:0.10.2 as builder
+FROM phusion/baseimage:0.11 as builder
 LABEL maintainer="chevdor@gmail.com"
 LABEL description="This is the build stage for Substrate. Here we create the binary."
 
@@ -26,7 +26,7 @@ RUN curl https://sh.rustup.rs -sSf | sh -s -- -y && \
 
 # ===== SECOND STAGE ======
 
-FROM phusion/baseimage:0.10.2
+FROM phusion/baseimage:0.11
 LABEL maintainer="chevdor@gmail.com"
 LABEL description="This is the 2nd stage: a very small image where we copy the Substrate binary."
 ARG PROFILE=release