test: restore Dockerfile ENTRYPOINT for all test runs (fix Nix TLS on CentOS)

All test scripts (unit, integration, e2e) previously overwrote the Docker
ENTRYPOINT by using `--entrypoint bash`, which bypassed the container’s
startup logic in `docker-entry.sh`.

`docker-entry.sh` performs essential initialization steps such as:

- CA bundle auto-detection (NIX_SSL_CERT_FILE, SSL_CERT_FILE, etc.)
- Nix environment setup
- PATH adjustments and distro logging

By removing the explicit `--entrypoint bash` and invoking:

  bash -lc '...'

directly as the container command, the Dockerfile’s ENTRYPOINT is restored
and runs as intended before executing the test logic.

This fixes TLS issues in CentOS E2E runs where Nix was unable to fetch
flake inputs due to missing CA configuration.

https://chatgpt.com/share/693ac1f3-fb7c-800f-9e5c-b40c351a9f04

scripts/test/test-e2e.sh

@@ -15,9 +15,8 @@ for distro in $DISTROS; do
     -e PKGMGR_DEV=1 \
     -e TEST_PATTERN="${TEST_PATTERN}" \
     --workdir /src \
-    --entrypoint bash \
     "package-manager-test-${distro}" \
-    -c '
+    bash -lc '
       set -euo pipefail
 
       # Load distro info

scripts/test/test-integration.sh

@@ -14,9 +14,8 @@ docker run --rm \
   --workdir /src \
   -e PKGMGR_DEV=1 \
   -e TEST_PATTERN="${TEST_PATTERN}" \
-  --entrypoint bash \
   "package-manager-test-${distro}" \
-  -c '
+  bash -lc '
     set -e;
     git config --global --add safe.directory /src || true;
     nix develop .#default --no-write-lock-file -c \

scripts/test/test-unit.sh

@@ -14,9 +14,8 @@ docker run --rm \
   --workdir /src \
   -e PKGMGR_DEV=1 \
   -e TEST_PATTERN="${TEST_PATTERN}" \
-  --entrypoint bash \
   "package-manager-test-${distro}" \
-  -c '
+  bash -lc '
     set -e;
     git config --global --add safe.directory /src || true;
     nix develop .#default --no-write-lock-file -c \