Replace build arguments with docker/compose secrets

Docker's documentation [suggests](https://docs.docker.com/reference/dockerfile/#arg) not to use build arguments to pass secrets, so this change updates the `Dockerfile` to use [secret mounts](https://docs.docker.com/build/building/secrets/#secret-mounts), and the recommended way to [manage secrets in docker compose](https://docs.docker.com/compose/how-tos/use-secrets/).

Hackney.Shared.Tenure.Tests/Dockerfile

@@ -3,8 +3,6 @@ FROM mcr.microsoft.com/dotnet/sdk:6.0
 # disable microsoft telematry
 ENV DOTNET_CLI_TELEMETRY_OPTOUT='true'
 
-ARG LBHPACKAGESTOKEN
-ENV LBHPACKAGESTOKEN=$LBHPACKAGESTOKEN
 WORKDIR /app
 
 # Copy csproj and restore as distinct layers
@@ -13,8 +11,16 @@ COPY ./Hackney.Shared.Tenure/Hackney.Shared.Tenure.csproj ./Hackney.Shared.Tenur
 COPY ./Hackney.Shared.Tenure.Tests/Hackney.Shared.Tenure.Tests.csproj ./Hackney.Shared.Tenure.Tests/
 COPY /nuget.config /root/.nuget/NuGet/NuGet.Config
 
-RUN dotnet restore ./Hackney.Shared.Tenure/Hackney.Shared.Tenure.csproj
-RUN dotnet restore ./Hackney.Shared.Tenure.Tests/Hackney.Shared.Tenure.Tests.csproj
+# We mount secrets so they can't end up in logs or build layers.
+# We chain both restore commands so we only make the token available
+# once and don't store it elsewhere.
+# see:
+#   - https://docs.docker.com/reference/dockerfile/#arg
+#   - https://docs.docker.com/compose/how-tos/use-secrets/
+RUN --mount=type=secret,id=LBHPACKAGESTOKEN \
+  export LBHPACKAGESTOKEN=$(cat /run/secrets/LBHPACKAGESTOKEN) && \
+  dotnet restore ./Hackney.Shared.Tenure/Hackney.Shared.Tenure.csproj && \
+  dotnet restore ./Hackney.Shared.Tenure.Tests/Hackney.Shared.Tenure.Tests.csproj
 
 # Copy everything else and build
 COPY . .

docker-compose.yml

@@ -6,5 +6,12 @@ services:
     build:
       context: .
       dockerfile: Hackney.Shared.Tenure.Tests/Dockerfile
-      args:
-        - LBHPACKAGESTOKEN=${LBHPACKAGESTOKEN}
+      secrets:
+        - LBHPACKAGESTOKEN
+
+# see https://docs.docker.com/compose/how-tos/use-secrets/#build-secrets
+# Combines with a "secrets" block in each service to expose it as a file in
+# /run/secrets/, e.g. /run/secrets/LBHPACKAGESTOKEN
+secrets:
+  LBHPACKAGESTOKEN:
+    environment: LBHPACKAGESTOKEN