dockerfiles: upgrade to ubuntu:20.04; some chore (#3828)

* dockerfiles: upgrade to ubuntu:20.04; some chore

* dockerfiles: fq container names

polkadot/scripts/docker/staking-miner/staking_miner-builder.Dockerfile

@@ -14,7 +14,7 @@ RUN cargo build --locked --$PROFILE --package staking-miner
 
 # ===== SECOND STAGE ======
 
-FROM debian:buster-slim
+FROM docker.io/library/ubuntu:20.04
 LABEL description="This is the 2nd stage: a very small image where we copy the binary."
 LABEL io.parity.image.authors="devops-team@parity.io" \
 	io.parity.image.vendor="Parity Technologies" \
@@ -43,4 +43,4 @@ ENV RUST_LOG="info"
 # check if the binary works in this container
 RUN /usr/local/bin/staking-miner --version
 
-ENTRYPOINT [ "/usr/local/bin/staking-miner"]
+ENTRYPOINT [ "/usr/local/bin/staking-miner" ]

polkadot/scripts/docker/staking-miner/staking_miner-injected.Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:buster-slim
+FROM docker.io/library/ubuntu:20.04
 
 # metadata
 ARG VCS_REF
@@ -22,6 +22,7 @@ RUN apt-get update && \
 	DEBIAN_FRONTEND=noninteractive apt-get install -y \
 		libssl1.1 \
 		ca-certificates && \
+# apt cleanup
 	apt-get autoremove -y && \
 	apt-get clean && \
 	find /var/lib/apt/lists/ -type f -not -name lock -delete; \