feat: control reading `/run/secrets` via USE_SECRETS_DIR env var

[akrantz01]

What type of PR is this?

• feature

What this PR does / why we need it:

Adds a new environment variable called USE_SECRETS_DIR that controls whether secrets are loaded from /run/secrets. It is enabled by default to maintain the existing behaviour.

This prevents issues in non-Docker environments where /run/secrets may not exist or be readable by the current user.

Which issue(s) this PR fixes:

Fixes #3852

Testing

For each scenario, a new dev container environment was created.

/run/secrets is readable and contains a file for LDAP_SERVER_URL:

• verified LDAP_SERVER_URL is populated when USE_SECRETS_DIR=true or is not explicitly set
• verified LDAP_SERVER_URL is not populated when USE_SECRETS_DIR=false

/run/secrets is not readable (follow instructions in issue) and contains a file for LDAP_SERVER_URL:

• verified task py raises the error from the issue when USE_SECRETS_DIR=true or is not explicitly set
• verified LDAP_SERVER_URL is not populated when USE_SECRETS_DIR=false

/run/secrets does not exist:

• verified a Pydantic warning is raised when USE_SECRETS_DIR=true or is not explicitly set
• verified no warning is raised when USE_SECRETS_DIR=false

[michael-genson]

This issue should be addressed by https://github.com/mealie-recipes/mealie/pull/4002

Happy to look at this one if you think it offers something not addressed by https://github.com/mealie-recipes/mealie/pull/4002, if you could confirm either way

[akrantz01]

Nope, that's great to see #4002 merged!