Upgrade base to ubuntu 20.04

The noninteractive env var had to be added since tzdata would try to ask
for a region.
ADDs changed to COPYs as it is good practice in the docker community.

Also:
- Remove fixed GCC_VER constraint (7.2.0 is not supported anymore)
- Use rust musl make 0.9.9 (patch level update)

Author: asaaki

Dockerfile

@@ -1,10 +1,12 @@
-FROM ubuntu:16.04
+FROM ubuntu:20.04
 
 # The Rust toolchain to use when building our image
 ARG TOOLCHAIN=stable
 ARG TARGET=x86_64-unknown-linux-musl
 ARG OPENSSL_ARCH=linux-x86_64
+ARG RUST_MUSL_MAKE_VER=0.9.9
 
+ENV DEBIAN_FRONTEND=noninteractive
 ENV RUST_MUSL_CROSS_TARGET=$TARGET
 
 # Make sure we have basic dev tools for building C libraries.  Our goal
@@ -26,15 +28,14 @@ RUN apt-get update && \
     apt-get clean && rm -rf /var/lib/apt/lists/*
 
 # Install cross-signed Let's Encrypt R3 CA certificate
-ADD lets-encrypt-r3-cross-signed.crt /usr/local/share/ca-certificates
+COPY lets-encrypt-r3-cross-signed.crt /usr/local/share/ca-certificates
 RUN update-ca-certificates
 
-ADD config.mak /tmp/config.mak
-RUN cd /tmp && \
-    curl -Lsq -o musl-cross-make.zip https://github.com/richfelker/musl-cross-make/archive/v0.9.8.zip && \
+COPY config.mak /tmp/config.mak
+RUN cd /tmp && curl -Lsq -o musl-cross-make.zip https://github.com/richfelker/musl-cross-make/archive/v$RUST_MUSL_MAKE_VER.zip && \
     unzip -q musl-cross-make.zip && \
     rm musl-cross-make.zip && \
-    mv musl-cross-make-0.9.8 musl-cross-make && \
+    mv musl-cross-make-$RUST_MUSL_MAKE_VER musl-cross-make && \
     cp /tmp/config.mak /tmp/musl-cross-make/config.mak && \
     cd /tmp/musl-cross-make && \
     TARGET=$TARGET make install -j 4 > /tmp/musl-cross-make.log && \

config.mak

@@ -15,10 +15,9 @@ OUTPUT = /usr/local/musl
 # Setting a blank version for linux will suppress installation of kernel
 # headers, which are not needed unless compiling programs that use them.
 
-# BINUTILS_VER = 2.25.1
- GCC_VER = 7.2.0
-
-# MUSL_VER = 1.1.19
+# BINUTILS_VER =
+# GCC_VER =
+# MUSL_VER =
 # GMP_VER =
 # MPC_VER =
 # MPFR_VER =