Bug: Docker Secrets lead to uncaughtException

As soon as you use docker secrets, you will get the following exception:

```
error: uncaughtException: Cannot freeze array buffer views with elements 15:29:21 [7/7167]TypeError: Cannot freeze array buffer views with elements at Function.freeze () at deepFreeze (/home/hackmd/app/node_modules/deep-freeze/index.js:2:10) at /home/hackmd/app/node_modules/deep-freeze/index.js:9:7 at Array.forEach () at deepFreeze (/home/hackmd/app/node_modules/deep-freeze/index.js:4:33) at /home/hackmd/app/node_modules/deep-freeze/index.js:9:7 at Array.forEach ()
at deepFreeze (/home/hackmd/app/node_modules/deep-freeze/index.js:4:33)
at Object. (/home/hackmd/app/lib/config/index.js:214:10)
at Module._compile (node:internal/modules/cjs/loader:1198:14)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1252:10)
at Module.load (node:internal/modules/cjs/loader:1076:32)
at Function.Module._load (node:internal/modules/cjs/loader:911:12)
at Module.require (node:internal/modules/cjs/loader:1100:19)
at require (node:internal/modules/cjs/helpers:119:18)
at Object. (/home/hackmd/app/app.js:23:14)
at Module._compile (node:internal/modules/cjs/loader:1198:14)
```

If you use random secrets, that are not used by https://github.com/hackmdio/codimd/blob/develop/lib/config/dockerSecret.js, there is no exception. If you used a named secret (e.g. `imgur_clientid`), like in the following example, you get the exception.

```
codimd:
image: hackmdio/hackmd:2.6.1
container_name: codimd
[... other container settings ...]
secrets:
- source: codimd_imgur_secret
target: imgur_clientid # when target = "abcef" no exception, but the secret is not used of course
```