[FR] Support running Docker image as non-root user

**Internal/External**
*External* otherwise. (Midnight)

**Area**
*Other* Any other topic (Delegation, Ranking, ...).

**Summary**
The run-node script unconditionally writes to /usr/local/bin/env, a root-owned path inside the container filesystem, making it impossible to run the container as a non-root user. Additionally, when using the config merge feature, the entrypoint script writes to /opt/cardano/config//, which also fails as non-root.

**Steps to reproduce**
1. Run ghcr.io/intersectmbo/cardano-node:10.5.2 with a non-root security context:
securityContext:
runAsNonRoot: true
runAsUser: 1001
runAsGroup: 1001
2. Container crashes with:
/usr/local/bin/run-node: line 91: /usr/local/bin/env: Permission denied
3. If CARDANO_CONFIG_JSON_MERGE is also set, the entrypoint additionally fails with:
/usr/local/bin/entrypoint: line 31: /opt/cardano/config/preview/config-merged.json: Permission denied

**Expected behavior**
The container should support running as a non-root user. Generated files should be written to a user-writable location (e.g. /tmp) instead of root-owned paths:
- run-node writeRootEnv() (line 91): write to /tmp/env instead of /usr/local/bin/env
- entrypoint (line 31): write config-merged.json / topology-merged.json to /tmp instead of /opt/cardano/config//

**System info (please complete the following information):**
- OS Name: Linux (Kubernetes)
- OS Version: Kernel 5.10
- Node version: cardano-node 10.5.2 (ghcr.io/intersectmbo/cardano-node:10.5.2)
- CLI version: N/A