remove entrypoint from the linux image

Current Rustwide versions don't rely on MAP_USER_ID anymore, so the
entrypoint does not add anything of value.

Author: pietroalbini

Diff for: linux/Dockerfile

@@ -2,17 +2,8 @@ FROM ubuntu:focal
 
 # Install the packages contained in `packages.txt`
 COPY packages.txt /opt/crates-build-env/packages.txt
-RUN apt-get update && cat /opt/crates-build-env/packages.txt | DEBIAN_FRONTEND=noninteractive xargs apt-get install -y
-
-# Fix the "sudo: setrlimit(RLIMIT_CORE): Operation not permitted" error when
-# starting up the container with `MAP_USER_ID`. Upstream issue:
-#
-#    https://github.com/sudo-project/sudo/issues/42
-#
-RUN echo "Set disable_coredump false" >> /etc/sudo.conf
-
-# Setup the entrypoint
-COPY entrypoint.sh /opt/crates-build-env/entrypoint.sh
-ENTRYPOINT ["/opt/crates-build-env/entrypoint.sh"]
+RUN apt-get update && \
+    cat /opt/crates-build-env/packages.txt | DEBIAN_FRONTEND=noninteractive xargs apt-get install -y && \
+    rm -rf /var/lib/apt/lists/*
 
 CMD ["bash"]