Openshift 4 ConfigMap binary file is mounted as directory - kubernetes

Question

I have created ConfigMap from Openshift 4 Windows 10 CLI:
.\oc create configmap my-cacerts --from-file=cacerts
I can see ConfigMap with name my-cacerts and download binary file cacerts from it using web interface of Openshift 4
Now I mount it (part of my-deployment.yaml)
containers:
volumeMounts:
- name: my-cacerts-volume
mountPath: /etc/my/cacerts
volumes:
- name: my-cacerts-volume
config-map:
name: my-cacerts
Unfortunately /etc/my/cacerts is mounted as a empty folder but not as a single binary file.
How can I mount cacerts as a file and not as a directory?
Update:
If I issue
.\oc get configmap my-cacerts
There is following output:
apiVersion: v1
binaryData:
cacerts: ... big long base64...
kind: ConfigMap
metadata: ...
If I issue
.\oc describe pod my-pod
I get
Volumes:
my-cacerts-volume:
Type: EmptyDir (a temporary directory that shares a pod's lifetime)

Answer 1

Your volumes definition is incorrect, config-map does not exist and is invalid, but it seems the API is silently falling back to an EmptyDir here, thus leading to an empty directory.
When you create a ConfigMap using the oc command above, the result will be a ConfigMap that looks like this (note that there is one key called "cacerts"):
apiVersion: v1
kind: ConfigMap
metadata:
name: my-cacerts
data:
cacerts: |
Hello world!
In the volumes section, then use configMap: together with subPath as follows to mount a only a single key ("cacerts") from your ConfigMap:
$ oc edit deployment my-deployment
[..]
spec:
containers:
- image: registry.fedoraproject.org/fedora-minimal:33
name: fedora-minimal
volumeMounts:
- mountPath: /etc/my/cacerts
name: my-cacerts-volume
subPath: cacerts
[..]
volumes:
- configMap:
name: my-cacerts
defaultMode: 420
name: my-cacerts-volume
This then results in:
$ oc rsh ...
sh-5.0$ ls -l /etc/my/cacerts
-rw-r--r--. 1 root 1000590000 13 Dec 3 19:11 /etc/my/cacerts
sh-5.0$ cat /etc/my/cacerts
Hello world!
You can also leave subPath out and set /etc/my/ as the destination for the same result, as for each key there will be a file:
[..]
volumeMounts:
- mountPath: /etc/my/
name: my-cacerts-volume
[..]
volumes:
- configMap:
name: my-cacerts
name: my-cacerts-volume
For the right syntax, you can also check the documentation

Answer 2

For Openshift 4 defaultMode should be specified:
volumeMounts:
- mountPath: /etc/my
name: cacerts-ref
readOnly: true
volumes:
- name: cacerts-ref
configMap:
defaultMode: 420
name: cacerts
After that configMap contents are mapped correctly.
.\oc describe pod my-pod
Volumes:
cacerts-ref:
Type: ConfigMap (a volume populated by a ConfigMap)
Name: cacerts
Optional: false