Issue:
Bug: [Docker] Change user to non-privileged one before running binary



We should not be running binary at highest privileges. It is better to not use `app:app` instead.

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->

This PR modifies the `Dockerfile` to specify the user at runner stage.

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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From the security perspective, it is better to be a `user` rather than being a `root`.
Closes https://github.com/juspay/hyperswitch/issues/7643

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->

build succeeded (tested by making a debug build):

![image](https://github.com/user-attachments/assets/3c27cbe1-56fa-48b9-a4bb-4a5ebb59563c)

```sh
podman build \
    --load \
    --file Dockerfile \
    --build-arg "BINARY=router" \
    --tag hyperswitch-router \
    .
```

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:


build succeeded (tested by making a debug build):

![image](https://github.com/user-attachments/assets/3c27cbe1-56fa-48b9-a4bb-4a5ebb59563c)

```sh
podman build \
    --load \
    --file Dockerfile \
    --build-arg "BINARY=router" \
    --tag hyperswitch-router \
    .
```

Patch:
diff --git a/Dockerfile b/Dockerfile
index 4914e6ad235..b2ad0809978 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,6 +72,10 @@ RUN mkdir -p ${BIN_DIR}
 
 COPY --from=builder /router/target/release/${BINARY} ${BIN_DIR}/${BINARY}
 
+# Create the 'app' user and group
+RUN useradd --user-group --system --no-create-home --no-log-init app
+USER app:app
+
 WORKDIR ${BIN_DIR}
 
 CMD ./${BINARY}