Why does the bash script in my docker build file return corrupt json?

Why is docker build corrupting the JSON in $VSS_NUGET_EXTERNAL_FEED_ENDPOINTS?

I am using this article (spefically example 2 which uses Buildkit and docker build secrets) to help populate my Dockerfile. The process will:

• pass a PAT during docker build,
• mount the PAT as a secret,
• export an escaped JSON string using the secret (via cat /pat) to ENV, and
  • ^^^ the above JSON is corrupt ^^^
• use the env variable to call a NuGet private feed before
• making a dotnet restore request.

The bash command in question is:

RUN --mount=type=secret,id=pat,dst=/pat export VSS_NUGET_EXTERNAL_FEED_ENDPOINTS="{\"endpointCredentials\": [{\"endpoint\":\"https://<my private feed URL>/nuget/v3/index.json\", \"username\":\"docker\", \"password\":\"`cat /pat`\"}]}" && \
  echo $VSS_NUGET_EXTERNAL_FEED_ENDPOINTS

Output of the echo command (notice the trailing end of the PAT is appended AFTER the JSON closing braces):

{"endpointCredentials": [{"endpoint":"https://<my private feed URL>/nuget/v3/index.json", "username":"docker", "password":"qwak...jiq5"}]}vtzkzv4a

When I run the command in bash (in Windows Subsystem for Linux) it works just fine:

~ echo "my-pat-token" >> /pat
~ export MY_ENV_VAR="{\"endpointCredentials\": [{\"endpoint\":\"https://<URL of private feed>/nuget/v3/index.json\", \"username\":\"build\", \"password\":\"`cat /pat`\"}]}"
~ echo $MY_ENV_VAR
{"endpointCredentials": [{"endpoint":"https://<URL of private feed>/nuget/v3/index.json", "username":"build", "password":"my-pat-token"}]}

Dockerfile:

# syntax=docker/dockerfile:1.2

FROM mcr.microsoft.com/dotnet/aspnet:3.1 AS base
WORKDIR /app
EXPOSE 443

FROM mcr.microsoft.com/dotnet/sdk:3.1 AS build
WORKDIR /src

# Install NuGet credential provider
RUN apt-get update && apt-get install -y locales \
   && sed -i -e 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/' /etc/locale.gen \
   && dpkg-reconfigure --frontend=noninteractive locales && update-locale LANG=en_US.UTF-8 \
   # Download the artifact credential provider
   && wget -qO- https://raw.githubusercontent.com/Microsoft/artifacts-credprovider/master/helpers/installcredprovider.sh | bash

COPY NuGet.Config .

COPY ["src/Discovery.Api/Discovery.Api.csproj", "src/Discovery.Api/"]
COPY ["src/Discovery.Service/Discovery.Service.csproj", "src/Discovery.Service/"]
COPY ["src/Discovery.Data/Discovery.Data.csproj", "src/Discovery.Data/"]

# Use the secret to set the credential provider variable and run restore.
ENV NUGET_CREDENTIALPROVIDER_SESSIONTOKENCACHE_ENABLED true
RUN --mount=type=secret,id=pat,dst=/pat export VSS_NUGET_EXTERNAL_FEED_ENDPOINTS="{\"endpointCredentials\": [{\"endpoint\":\"https://<my private feed URL>/nuget/v3/index.json\", \"username\":\"docker\", \"password\":\"`cat /pat`\"}]}" && \
  echo $VSS_NUGET_EXTERNAL_FEED_ENDPOINTS

RUN dotnet restore "src/Discovery.Api/Discovery.Api.csproj" --configfile NuGet.Config

COPY . .
WORKDIR "/src/Discovery.Api"
RUN dotnet build "src/Discovery.Api/Discovery.Api.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "src/Discovery.Api/Discovery.Api.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "src/Discovery.Api/Discovery.Api.dll"]

docker build command:

docker build --no-cache --progress=plain --secret id=pat,src=./pat -t discovery-api:dev -f src\Discovery.Api\Dockerfile .

docker --version

Docker version 20.10.8, build 3967b7d

I am running docker on linux containers.