Q:

Windows GitLab CI Runner using Bash

I'm trying to use bash as the shell on Windows for a GitLab CI Runner.
concurrent = 1
check_interval = 0

[[runners]]
  name = "DESKTOP-RQTQ13S"
  url = "https://example.org/ci"
  token = "fooooooooooooooooooobaaaaaaaar"
  executor = "shell"
  shell = "bash"
  [runners.cache]

Unfortunately I can not find an option to specify the actual shell program that the CI Runner should use. By default, it just tries to run bash which it can not find. I don't know why, because when I open up a Windows command line and enter bash it works.
Running with gitlab-ci-multi-runner 1.9.4 (8ce22bd)
Using Shell executor...
ERROR: Build failed (system failure): Failed to start process: exec: "bash": executable file not found in %PATH%

I tried adding a file bash.cmd to my user directory containing
@"C:\Program Files\Git\usr\bin\bash.exe" -l

That gives me this strange error:
Running with gitlab-ci-multi-runner 1.9.4 (8ce22bd)
Using Shell executor...
Running on DESKTOP-RQTQ13S...
/usr/bin/bash: line 43: /c/Users/niklas/C:/Users/niklas/builds/aeb38de4/0/niklas/ci-test.tmp/GIT_SSL_CAINFO: No such file or directory
ERROR: Build failed: exit status 1

Is there a way to properly configure this?

A:

There are two issues going on here, and both can probably be solved.

gitlab-runner cannot find bash
gitlab-runner doesn't combine unix-style and Windows-style paths very well. 

You have essentially succeeded in solving the first one by creating the bash.cmd file. But if you're curious about why it didn't work without it, my guess is that bash runs in your command prompt because the directory that contains it (e.g. in your case "C:\Program Files\Git\usr\bin") is included in the PATH environment variable for your user account. But perhaps you are running the gitlab-runner in the system account, which might not have the same PATH.
So the first thing to do is just check your system's PATH variable and add the bin directory if necessary (i.e. using the System applet in the Control Panel as described here or here). Just make sure you restart your machine after you make the change, because the change isn't applied until after you restart. That should make bash work, even when called from a service running in the system or admin account.
As for the strange error you got after creating bash.cmd, that was due to the second issue. Paths are often really hard to get right when combining bash and Windows. Gitlab-runner is probably trying to determine whether the build path is relative or absolute, and ends up prepending the windows path with what it thinks is the working directory ($PWD). This looks like a bug, but gitlab still has not fixed it (as of version 9.0 of the runner!!) and probably never will. Maybe they have decided it is not a bug or that it is due to bugs in underlying software or tools that they can't fix or that it would be too difficult to fix. Anyway, I've discovered a work-around.  You can specify the base path for builds in the config.toml file. If you use a unix-style path, it fixes the problem.
On windows, config.toml is usually in the same folder as your gitlab-runner.exe (or gitlab-multi-runner-amd64.exe etc). Open that file in your favorite text editor. Then find the [[runners]] section and add two lines similar to the following.
builds_dir="/c/gitlab-runner/builds/"
cache_dir="/c/gitlab-runner/cache/"

The path you use should be the "bash version" of whatever directory you want gitlab-runner to use for storing builds etc. Importantly if you are using cygwin, you would use a path similar to /cygdrive/c/... instead of just /c/...  (which is appropriate for msys-git or standalone MSYS2 etc).
Here's an example of a config.toml file:
[[runners]]
  name = "windows"
  url = "https://your.server.name"
  token = "YOUR_SECRET_TOKEN"
  executor = "shell"
  shell = "bash"
  builds_dir="/c/gitlab-runner/builds/"
  cache_dir="/c/gitlab-runner/cache/"

