GH-47970: [CI][C++] Fix a bug that JNI jobs runs nothing

[kou]

Rationale for this change

If we use bash -i -c as ENTRYPOINT, we need to pass command line as a string such as archery docker run cpp-jni 'ls -la' not archery docker run cpp-jni ls -lah.

Our compose.yaml assumes the latter style. So our command line in compose.yaml is ignored.

What changes are included in this PR?

Remove needless SHELL and ENTRYPOINT.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [CI][C++] JNI jobs build nothing #47970

[github-actions]

⚠️ GitHub issue #47970 has been automatically assigned in GitHub to PR creator.

[kou]

Hmm... Boost detection is failing:

https://github.com/apache/arrow/actions/runs/18862133948/job/53822393392?pr=47972#step:6:2833

CMake Warning at /usr/local/share/cmake-3.29/Modules/FindBoost.cmake:2234 (message):
  No header defined for process; skipping header check (note: header-only
  libraries have no designated component)
Call Stack (most recent call first):
  /opt/vcpkg/scripts/buildsystems/vcpkg.cmake:898 (_find_package)
  cmake_modules/ThirdpartyToolchain.cmake:310 (find_package)
  cmake_modules/ThirdpartyToolchain.cmake:1287 (resolve_dependency)
  CMakeLists.txt:523 (include)


CMake Error at /usr/local/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Boost (missing: Boost_INCLUDE_DIR filesystem system)
  (Required is at least version "1.64")
Call Stack (most recent call first):
  /usr/local/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake-3.29/Modules/FindBoost.cmake:2393 (find_package_handle_standard_args)
  /opt/vcpkg/scripts/buildsystems/vcpkg.cmake:898 (_find_package)
  cmake_modules/ThirdpartyToolchain.cmake:310 (find_package)
  cmake_modules/ThirdpartyToolchain.cmake:1287 (resolve_dependency)
  CMakeLists.txt:523 (include)


-- Configuring incomplete, errors occurred!