Backup SQL Server 2019 in a Docker Linux container fails - sql-server

Question

Attempted multiple ways to do the backup.
Tried adding SA as a root user in the container
Azure Data studio
BACKUP DATABASE [PrestigeCars] TO DISK = N'/var/opt/mssql/backup//PrestigeCars-202044-8-4-52.bak' WITH NOFORMAT, NOINIT, NAME = N'PrestigeCars--2020-04-04T12:04:52', NOSKIP, REWIND, NOUNLOAD, STATS = 10
Msg 3201, Level 16, State 1, Line 1
Cannot open backup device '/var/opt/mssql/backup//PrestigeCars-202044-8-4-52.bak'. Operating system error 2(The system cannot find the file specified.).
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.
Total execution time: 00:00:00.217
SSMS
backup database [PrestigeCars]
to disk = N'/var/opt/mssql/backup//PrestigeCars-202044-6-42-9.bak'
with noformat
, noinit
, name = N'PrestigeCars--20200404'
, noskip
, rewind
, nounload
, compression
, stats = 10;
SQLCMD
sqlcmd -S localhost,12001 -U SA -Q "BACKUP DATABASE [PrestigeCars] TO DISK = N'/var/opt/mssql/backup/CSCI331-Backup/PrestigeCars-202044-6-42-9.bak' WITH NOFORMAT, NOINIT, NAME = 'PrestigeCars-20200404', SKIP, NOREWIND, NOUNLOAD, STATS = 10"

Answer 1

Thanks to all of those of you that have helped. The issue was not missing folders but a permissions issue.
The folders were initially create when copying backups to the container:
docker cp /Users/YourUsername/CSCI331-Backup/TSQLV4.bak linux-sql2k19:/var/opt/mssql/backup/
*The permissions were 4 drwxr-xr-**x 3 root root 4096 Jan 31 19:06 backup***
I tried various Ubuntu create sudo adduser but none of the commands sudo or apt-get worked. (https://help.ubuntu.com/community/FilePermissions)
I found this command to connect as the root user
docker exec -it -u root 874 bash
cd /var/opt/mssql/backup/
cd ..
chmod 777 backup
cd ..
chmod 777 mssql
cd ..
chmod 777 opt
cd ..
chmod 777 var
Close the container. I backed up databases in SSMS and Azure Data Studio. Double Yeah!

Answer 2

Directory /var/opt/mssql/backup does not exist in standard SQL Server Linux images. You'll need to first create the directory by running the following command in the container:
mkdir /var/opt/mssql/backup
Also, as #Larnu pointed out, you have an extra backslash in the path. The backup command should be:
BACKUP DATABASE [PrestigeCars] TO DISK = N'/var/opt/mssql/backup/PrestigeCars-202044-8-4-52.bak' WITH NOFORMAT, NOINIT, NAME = N'PrestigeCars--2020-04-04T12:04:52', NOSKIP, REWIND, NOUNLOAD, STATS = 10;