plugincontainer/ci: fix docker build

Install adduser, specify ubuntu image version

Author: tvoran

plugincontainer/examples/container/Dockerfile

@@ -1,17 +1,17 @@
 # Copyright (c) HashiCorp, Inc.
 # SPDX-License-Identifier: MPL-2.0
 
-FROM gcr.io/distroless/static-debian12 as root
+FROM gcr.io/distroless/static-debian12 AS root
 
 COPY go-plugin-counter /bin/go-plugin-counter
 
 ENTRYPOINT [ "/bin/go-plugin-counter" ]
 
-FROM docker.mirror.hashicorp.services/ubuntu as nonroot
+FROM docker.mirror.hashicorp.services/ubuntu:24.04 AS nonroot
 
 COPY go-plugin-counter /bin/go-plugin-counter
 
-RUN apt-get update && apt-get install -y libcap2-bin acl && \
+RUN apt-get update && apt-get install -y libcap2-bin acl adduser && \
     addgroup --system nonroot && \
     adduser --system --ingroup nonroot nonroot && \
     chown -R nonroot:nonroot /bin/go-plugin-counter && \
@@ -22,9 +22,9 @@ USER nonroot
 
 ENTRYPOINT [ "/bin/go-plugin-counter" ]
 
-FROM nonroot as nonroot-mlock
+FROM nonroot AS nonroot-mlock
 
 ENTRYPOINT [ "/bin/go-plugin-counter-mlock" ]
 
 # Set root as the default image.
-FROM root
+FROM root

plugincontainer/examples/container/README.md

@@ -11,7 +11,7 @@ To run the example:
 
 ```sh
 # Build the plugin container image
-$ go build -o go-plugin-counter ./plugin-counter && docker build -t go-plugin-counter .
+$ go build -o go-plugin-counter ./plugin-counter && docker build -t plugin-counter .
 
 # Read and write
 $ go run main.go increment hello 1