Publish update-deps with rid

[mthalman]

The following exception occurs when update-dependencies attempts to update an existing PR:

System.TypeInitializationException: The type initializer for 'LibGit2Sharp.Core.NativeMethods' threw an exception.
 ---> System.DllNotFoundException: Unable to load shared library 'git2-106a5f2' or one of its dependencies. In order to help diagnose loading problems, consider setting the LD_DEBUG environment variable: libgit2-106a5f2: cannot open shared object file: No such file or directory
   at LibGit2Sharp.Core.NativeMethods.git_libgit2_init()
   at LibGit2Sharp.Core.NativeMethods.InitializeNativeLibrary()
   at LibGit2Sharp.Core.NativeMethods..cctor()
   --- End of inner exception stack trace ---
   at LibGit2Sharp.Core.NativeMethods.git_clone(git_repository*& repo, String origin_url, FilePath workdir_path, GitCloneOptions& opts)
   at LibGit2Sharp.Core.Proxy.git_clone(String url, String workdir, GitCloneOptions& opts)
   at LibGit2Sharp.Repository.Clone(String sourceUrl, String workdirPath, CloneOptions options)
   at Dotnet.Docker.Program.UpdateExistingPullRequest(GitHubAuth gitHubAuth, PullRequestOptions prOptions, String commitMessage, GitHubBranch upstreamBranch) in /update-dependencies/Program.cs:line 188
   at Dotnet.Docker.Program.CreatePullRequestAsync() in /update-dependencies/Program.cs:line 155
   at Dotnet.Docker.Program.Main(String[] args) in /update-dependencies/Program.cs:line 53

This issue is described in libgit2/libgit2sharp#1798. The workaround is to publish with the specific RID which provides the correct library. I also took this opportunity to update the Dockerfile to be based on Alpine which simplifies the content and produces a smaller image size.

There was also an issue with the update-dependencies logic where it would end up excluding all directories starting with . when copying the repo. This resulted in the exclusion of the .github folder which would end up deleting the files in that folder as part of the PR update.

Fixes #1898