Dockerfile.buildenv: reduce size by installing sudo along with other packages

We didn't clear the package cache again after the additional sudo install, which increased the size of the container.

docker/Dockerfile.buildenv

@@ -33,6 +33,7 @@ RUN apt-get update \
 	python3-pip \
 	rsync \
 	scons \
+	sudo \
 	unzip \
 	wget \
 	nano \
@@ -60,7 +61,5 @@ ARG USER_GID=$USER_UID
 RUN groupadd --gid $USER_GID $USERNAME \
  && useradd --uid $USER_UID --gid $USER_GID -m $USERNAME --shell /bin/bash \
 # Add sudo support for this user and remove the need to type in password.
- && apt-get update \
- && apt-get install -y sudo \
  && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME \
  && chmod 0440 /etc/sudoers.d/$USERNAME