Avoid KoreBuild issues when using dockerbuild.sh

- avoid MSB4011 warnings (about repeated Directory.Build.targets imports)
    - place `$DOTNET_HOME` beside repo root, not under it
- cherry-picked from #30729 internal branch for image update

nits:
- use current Docker image for 2.1 alpine
- set `$LANGUAGE` with other locale-related environment variables

build/docker/alpine.Dockerfile

@@ -1,4 +1,4 @@
-FROM microsoft/dotnet:2.1.0-preview1-runtime-deps-alpine
+FROM mcr.microsoft.com/dotnet/runtime-deps:2.1-alpine
 ARG USER
 ARG USER_ID
 ARG GROUP_ID
@@ -18,10 +18,14 @@ RUN apk add --no-cache \
 
 USER $USER_ID:$GROUP_ID
 
+# Use a location for .dotnet/ that's not under repo root.
+ENV DOTNET_HOME /code/.dotnet
+
 # Disable the invariant mode (set in base image)
 ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT false
 ENV LC_ALL en_US.UTF-8
 ENV LANG en_US.UTF-8
+ENV LANGUAGE en_US.UTF-8
 
 # Skip package initilization
 ENV DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1

dockerbuild.sh

@@ -90,6 +90,9 @@ fi
 dockerfile="$DIR/build/docker/$image.Dockerfile"
 tagname="aspnetcore-build-$image"
 
+# Use a location for .dotnet/ that's not under repo root in the container and don't reuse host's folder.
+mkdir "$(dirname $DIR)/.dotnet-$image"
+
 docker build "$(dirname "$dockerfile")" \
     --build-arg "USER=$(whoami)" \
     --build-arg "USER_ID=$(id -u)" \
@@ -114,6 +117,7 @@ docker run \
     -e PB_ASSETROOTURL \
     -e PRODUCTBUILDID \
     -v "$DIR:/code/build" \
+    -v "$(dirname $DIR)/.dotnet-$image:/code/.dotnet" \
     ${docker_args[@]+"${docker_args[@]}"} \
     $tagname \
     ./build.sh \