Add missing quotations in Dockerfile example

[mohammad7t]

Docker build raises error without those quotations.

[aviatesk]

@mhsekhavat Thanks for your contribution !
Could you tell me your OS ?

I think this may cause something like this.
In short, the need for quotations may depend on an user's OS.

[mohammad7t]

@aviatesk My OS is Debian Linux.

As docker documentation says, exec form will be parsed as JSON:

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

[wadethestealth]

@mhsekhavat thanks for the contribution

[aviatesk]

@mhsekhavat
Thanks for your description and contributions ! 😉