docker: Switch to official Rust image as base and rework

1. Use the official Rust nightly docker image as base
2. Remove hardcoded user
3. Use named volumes to cache .rustup and .cargo toolchain folders
    - Changing file permissions to user (chown) only needed on first launch
4. Cleanup apt folders after installing
5. Make bash the default fallback command
6. README.md: Unify workflows for Linux and MacOS

docker/entrypoint.sh

@@ -1,18 +1,29 @@
 #!/usr/bin/env bash
 
-# Use -e LOCAL_UID="$(id -u)" -e LOCAL_GID="$(id -g)"
-# on the docker run command line if the container build user is different
-# from the run user
+# Add local user
+# Either use LOCAL_UID and LOCAL_GID if passed in at runtime via
+# -e LOCAL_UID="$(id -u)" -e LOCAL_GID="$(id -g)" or fallback
+USER_NAME=redox
+RUN_UID=${LOCAL_UID:-9001}
+RUN_GID=${LOCAL_GID:-9001}
 
-CONT_UID=`id -u user`
-CONT_GID=`id -g user`
-RUN_UID=${LOCAL_UID:-$CONT_UID}
-RUN_GID=${LOCAL_GID:-$CONT_GID}
+groupadd --non-unique --gid $RUN_GID $USER_NAME
+useradd --non-unique --create-home --uid $RUN_UID --gid $USER_NAME --groups sudo $USER_NAME
 
-if [ $RUN_UID != $CONT_UID ] || [ $RUN_GID != $CONT_GID ]; then
+echo "$USER_NAME ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/user-no-sudo-password
+
+export HOME=/home/$USER_NAME
+
+# Check current UID and GID of files in the named volume caches for
+# cargo and rustup. Test only one of the top level folders to speed
+# things up.
+TESTFILE=$RUSTUP_HOME/settings.toml
+CACHED_UID=$(stat -c "%u" $TESTFILE)
+CACHED_GID=$(stat -c "%g" $TESTFILE)
+
+if [ $CACHED_UID != $RUN_UID ] || [ $RUN_GID != $CACHED_GID ]; then
     echo -e "\033[01;38;5;155mChanging user id:group to ${RUN_UID}:${RUN_GID}. Please wait...\033[0m"
-    groupmod -g $RUN_GID user
-    usermod -u $RUN_UID -g $RUN_GID user
+    chown $RUN_UID:$RUN_GID -R $CARGO_HOME $RUSTUP_HOME
 fi
 
-exec gosu user:user "$@"
+exec gosu $USER_NAME "$@"