Add `dotnet new` template for `.dockerignore` file

[lbussell]

Is your feature request related to a problem? Please describe.

When buliding a .NET Dockerfile, there are some directories that you should not copy into the Dockerfile's build context.
Currently, the best thing to do is visit the dotnet-docker samples and copy a known-good .dockerignore file from there.

Describe the solution you'd like

.NET should have a .dockerignore template built in. You should be able to run dotnet new dockerignore just like you run dotnet new gitignore in a new solution.

While this wouldn't make a difference for the SDK container publish feature, it would improve the UX when using Dockerfiles directly.

Additional context

Related: #16784 (comment)

[marcpopMSFT]

@baronfel Chet, thoughts on this? @lbussell do you have a proposal for the base contents of the file we should use?

We support editorconfig and gitignore so this one makes sense to me.

[baronfel]

It's slightly more complex because the directories you want to change differ based on if the repo is using Artifacts Path or not - the most basic template would work for most folks, but the long tail is long!

For comparison, here's what docker's own docker init experience gives for an ASP.NET Core application scaffolded through their system:

# Include any files or directories that you don't want to be copied to your
# container here (e.g., local build artifacts, temporary files, etc.).
#
# For more help, visit the .dockerignore file reference guide at
# https://docs.docker.com/go/build-context-dockerignore/

**/.DS_Store
**/.classpath
**/.dockerignore
**/.env
**/.git
**/.gitignore
**/.project
**/.settings
**/.toolstarget
**/.vs
**/.vscode
**/*.*proj.user
**/*.dbmdl
**/*.jfm
**/bin
**/charts
**/docker-compose*
**/compose.y*ml
**/Dockerfile*
**/node_modules
**/npm-debug.log
**/obj
**/secrets.dev.yaml
**/values.dev.yaml
LICENSE
README.md

Feels like anything we do should be at least that useful.

[lbussell]

here's what docker's own docker init experience gives for an ASP.NET Core application

Thanks for providing that extra context. Docker's example is quite a bit more comprehensive than our sample .dockerignore.

It's slightly more complex because the directories you want to change differ based on if the repo is using Artifacts Path or not - the most basic template would work for most folks, but the long tail is long!

Does the gitignore template handle that complexity today? I assume that it would face the same challenges. At first glance, it looks like it doesn't (although I am not familiar with the templating infra in the SDK): https://github.com/dotnet/sdk/blob/50f42240f56f2a15c1102100911a0d7f4ca4b6ff/template_feed/Microsoft.DotNet.Common.ItemTemplates/content/Gitignore/.gitignore