Update devcontainer definition (#138)

Author: swernli

.devcontainer/Dockerfile

@@ -1,14 +1,9 @@
-FROM mcr.microsoft.com/vscode/devcontainers/universal:1-linux
-
-USER root
-
-RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|apt-key add - && \
-    add-apt-repository "deb https://apt.llvm.org/focal/ llvm-toolchain-focal-13 main" && \
-    apt-get -y update && \
-    apt-get -y remove llvm-10 && \
-    apt-get -y install cmake clang-13 clang-format-13 clang-tidy-13 && \
+FROM mcr.microsoft.com/vscode/devcontainers/universal:linux
+RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add - && \
+    sudo add-apt-repository "deb https://apt.llvm.org/focal/ llvm-toolchain-focal-13 main" && \
+    sudo apt-get -y update && \
+    sudo apt-get -y remove llvm-10 && \
+    sudo apt-get -y install clang-13 clang-format-13 clang-tidy-13 && \
+    sudo apt autoremove -y && \
     npm install -g prettier@2.2.1
-
-USER codespace
-
 ENV CC=clang-13 CXX=clang++-13