Second stop on the Docker learning train - Dockerize an ASP.NET Core application

The second stop on my Docker learning train is a recommendation from the Docker Samples. The tutorial is titled 'Dockerize an ASP.NET Core application'.

When I first read through the tutorial, I figured this was going to be quick and easy. Due to making a few mistakes in creating the Dockerfile, I ended up with a few lessons learned.

The first lesson I learned was to read the output from running the docker build command carefully. It was interesting to see that the Docker daemon creates and removes intermediate containers.

The next lesson was when a container reports an error code 145 carefully double-check the Dockerfile. Pay close attention to what is being used for the DotNet Core runtime. The 'SDK' should be used for the restore and publish command while 'ASPNET' should be used for the runtime.

Another lesson learned is to use '.dockerignore' file to keep from unnecessary files from being included in the image.

The last lesson was to use interactive mode. The tutorial did not go through the steps I needed to do a bit of research to figure it out. The Alpine image does not include bash. I modified the Dockerfile to pull in bash and a few other tools. Rebuilt the image. The used the interactive mode switches to get a command prompt.

Dockerfile:

FROM mcr.microsoft.com/dotnet/core/sdk:3.1-alpine AS build
WORKDIR /app/aspnetapp

# copy csproj and restore as distinct layers
COPY myWebApp/*.csproj ./
RUN dotnet restore

# copy everything else and build app
COPY myWebApp/. ./
RUN dotnet publish -c Release -o out

# Build runtime image
FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-alpine AS runtime
WORKDIR /app
COPY --from=build /app/aspnetapp/out .

# Uncomment the following line to install bash and other tools useful for debugging
RUN apk add --no-cache bash coreutils grep sed

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

Command to build the image.

docker build -t {image name} .

Command to start the container in interactive mode and to override the default entry point

docker run -it --entrypoint /bin/bash {image name}

Reference

.dockerignore file

"Before the docker CLI sends the context to the docker daemon, it looks for a file named .dockerignore in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it. This helps to avoid unnecessarily sending large or sensitive files and directories to the daemon and potentially adding them to images using ADD or COPY." - https://docs.docker.com/engine/reference/#dockerignore-file