🚀 Containerizing a .NET 8 Application with SQL Server using Docker

GitHub Repository: https://github.com/musayyab-ali/GradLink_App

In this blog, I will walk you through how I containerized a .NET 8 application that connects to a SQL Server (MSSQL) database. The project was cloned from the above GitHub repository and customized to run smoothly inside Docker containers using a persistent volume for the database.

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🛠️ Prerequisites

• Docker installed on your machine

• .NET 8 SDK (for local development and testing)

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🧱 Step 1: Set Up MSSQL Database in Docker

We start by creating a SQL Server container using the following command:

docker run -e "ACCEPT_EULA=Y" -e "MSSQL_SA_PASSWORD=hassan@123" \
   -p 1433:1433 --name sql1 --hostname sql1 \
   -v sql1data:/var/opt/mssql \
   --restart unless-stopped \
   -d mcr.microsoft.com/mssql/server:2022-latest

🔍 What this does:

• Sets up SQL Server with a persistent volume sql1data

• Maps port 1433 to your local system for access

• Automatically restarts unless manually stopped

🧑‍💻 Step 2: Restore SQL Server Database from Backup

If you have an existing database backup file (e.g., newGradLink.bak), you can restore it to your SQL Server container using the following commands:

1. Check Backup Files
   Use sqlcmd to verify the contents of the backup file:

    sqlcmd -S localhost -U sa -P 'hassan@123' -Q "RESTORE FILELISTONLY FROM DISK = '/var/opt/mssql/backup/newGradLink.bak'"
   

2. Restore Database
   After verifying, restore the database using this command:

    sqlcmd -S localhost -U sa -P 'hassan@123' -Q "
    RESTORE DATABASE gradlink
    FROM DISK = '/var/opt/mssql/backup/newGradLink.bak'
    WITH MOVE 'GradLink' TO '/var/opt/mssql/data/gradlink.mdf',
         MOVE 'GradLink_log' TO '/var/opt/mssql/data/gradlink_log.ldf',
         REPLACE;"
   

📝 Note: Make sure the paths (e.g., /var/opt/mssql/backup/) in the SQL Server container are valid and the backup file is placed in the correct directory.

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⚙️ Step 3: Update Connection String in appsettings.json

Before building the .NET container, make sure your GradLink/ project’s appsettings.json contains the correct connection string:

{
  "ConnectionStrings": {
    "DefaultConnection": "Data Source=xxxxxxxx; Initial Catalog=GradLink; User ID=sa; Password=hassan@123; MultipleActiveResultSets=True;TrustServerCertificate=True; Integrated Security=False;"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

📝 Replace 172.21.114.18 with your Docker bridge IP or the IP of the SQL Server container if you’re in a custom network.

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📦 Step 4: Create the Dockerfile

Here's the Dockerfile used to build the .NET app container:

# Use .NET 8 SDK for build
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build

WORKDIR /app

# Copy csproj files and restore as distinct layers
COPY GradLink/GradLink.csproj GradLink/
COPY GradLink.Model/GradLink.Model.csproj GradLink.Model/
COPY GradLink.Repository/GradLink.Repository.csproj GradLink.Repository/

RUN dotnet restore GradLink/GradLink.csproj

# Copy everything else and build
COPY . .
RUN dotnet publish GradLink/GradLink.csproj -c Release -o out

# Runtime image
FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
WORKDIR /app
COPY --from=build /app/out .

ENTRYPOINT ["dotnet", "GradLink.dll"]

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🛠️ Step 5: Build and Run the .NET Container

✅ Build the Docker Image

cdocker build -t gradlink-app .

🚀 Run the Container and Link to SQL Server

 docker run -d -p 5000:8080 --name gradlink-app --link sql1 gradlink-app

• Application will be available at http://localhost:8080

• SQL Server is accessible inside the container as sql1

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🔄 Persistent Volume for SQL Server

The -v sql1data:/var/opt/mssql volume ensures that your database data persists even if the container restarts or is recreated.

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✅ Final Checklist

• ✅ MSSQL container up and running ✔️

• ✅ appsettings.json updated ✔️

• ✅ .NET app container built and running ✔️

• ✅ Persistent volume for DB ✔️

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🏁 Conclusion

You’ve successfully containerized a .NET 8 web application with a linked SQL Server instance. This setup is great for local development, testing, or preparing your app for cloud/container orchestration platforms like Kubernetes.

📝 Important Notes:

1. 🔗 Use a Custom Docker Network (Recommended)

To ensure both containers can talk to each other by name:

bashCopyEdit# Create a custom network
docker network create gradlink-network

# Run SQL Server container on this network
docker run -e "ACCEPT_EULA=Y" -e "MSSQL_SA_PASSWORD=hassan@123" \
   -p 1433:1433 --name sql1 --hostname sql1 \
   -v sql1data:/var/opt/mssql \
   --network gradlink-network \
   --restart unless-stopped \
   -d mcr.microsoft.com/mssql/server:2022-latest

# Run your .NET app container on the same network
docker run -d -p 8080:80 --name gradlink-app \
   --network gradlink-network \
   gradlink-app

This setup allows your .NET app to reach SQL Server via sql1 instead of needing an IP address.