Add 2.1 arm32v7 aspnetcore-runtime

[sdmaclea]

Fixes #480

@MichaelSimons @natemcmaster PTAL

[MichaelSimons]

Thanks Steve for the PR. A few more changes are necessary in order to merge. I apologize there isn't more doc on this.

1. Can you make this PR against the nightly branch? These will get merged into the official repo when RC1 is released.
2. You will need to modify the manifest.json to include entries for these new. This is what the builds and CI operate off of. Here is what I think would be needed off the top of my head:
   At https://github.com/dotnet/dotnet-docker/blob/master/manifest.json#L400 add:

            {
              "architecture": "arm",
              "dockerfile": "2.1/aspnetcore-runtime/stretch-slim/arm32v7",
              "os": "linux",
              "tags": {
                "2.1.0-rc1-aspnetcore-runtime-stretch-slim-arm32v7": {},
                "2.1-aspnetcore-runtime-stretch-slim-arm32v7": {}
              },
              "variant": "armv7"
            },

At https://github.com/dotnet/dotnet-docker/blob/master/manifest.json#L534, add:

        {
          "platforms": [
            {
              "architecture": "arm",
              "dockerfile": "2.1/aspnetcore-runtime/bionic/arm32v7",
              "os": "linux",
              "tags": {
                "2.1.0-rc1-aspnetcore-runtime-bionic-arm32v7": {},
                "2.1-aspnetcore-runtime-bionic-arm32v7": {}
              }
            }
          ]
        },

3. The unit tests should be updated to cover these new images. I am hopeful you can duplicate the 2.1 arm test cases and add , IsWeb = true
4. To update the readme, the https://github.com/dotnet/dotnet-docker/blob/master/scripts/TagsDocumentationTemplate.md should be updated and then you can run https://github.com/dotnet/dotnet-docker/blob/master/scripts/Get-TagsDocumentation.ps1 if you'd like. There are too many tags and links to keep straight :)

[MichaelSimons]

Are rc1 builds available? It would be nice to align the builds with amd64.

[sdmaclea]

I will match nightly version.

[sdmaclea]

I will close this and reopen when I have a nightly PR ready.