Windows shares use the SMB protocol. There are a couple of Go libraries for using SMB, but I have never used them so I cannot vouch for their utility. Here is one I Googled:

https://github.com/stacktitan/smb

Other options would be to ensure that the Windows share is mounted on the Linux host filesystem using cifs. Then you could just use the regular Go file utilities:

https://www.thomas-krenn.com/en/wiki/Mounting_a_Windows_Share_in_Linux

Or, you could install something like Cygwin on the Windows box and run an SSH server. This would allow you to use SCP:

https://godoc.org/github.com/tmc/scp

Before proceeding with debugging the GO code, it would be needed to do some "unskilled labour" within container in order to ensure pre-requisites are met:

1. samba client is installed and daemons are running;
2. the target name gets resolved;
3. there are no connectivity issues (routing, firewall rules, etc);
4. there are share access permissions;
5. mounting remote volume is allowed for the container.

Connect to the container:

$ docker ps 
$ docker exec -it container_id /bin/bash

Samba daemons are running:

$ smbd status
$ nmbd status

You use the right name format in your code and command lines:

UNC notation =>  \\server_name\share_name
URL notation =>  smb://server_name/share_name

Target name is resolvable

$ nslookup server_name.domain_name
$ nmblookup netbios_name 
$ ping server_name

Samba shares are visible

$ smbclient -L //server [-U user]       # list of shares

and accessible (ls, get, put commands provide expected output here)

$ smbclient //server/share
> ls 

Try to mount remote share as suggested by @cwadley (mount could be prohibited by default in Docker container):

$ sudo mount -t cifs -o username=geeko,password=pass //server/share /mnt/smbshare

For investigation purposes you might use the Samba docker container available at GitHub, or even deploy your application in it since it contains Samba client and helpful command line tools:

$ sudo docker run -it -p 139:139 -p 445:445 -d dperson/samba

After you get this working at the Docker level, you could easily reproduce this in Kubernetes.

You might do the checks from within the running Pod in Kubernetes:

$ kubectl get deployments --show-labels
$ LABEL=label_value; kubectl get pods -l app=$LABEL -o custom-columns=POD:metadata.name,CONTAINER:spec.containers[*].name
$ kubectl exec pod_name -c container_name -- ping -c1 server_name

Having got it working in command line in Docker and Kubernetes, you should get your program code working also.

Also, there is a really thoughtful discussion on StackOverflow regards Samba topic:
Mount SMB/CIFS share within a Docker container