fix(be): using npm when install pnpm on build image

[jspark2000]

Description

백엔드 API 도커이미지 빌드시에
pnpm을 corepack이 아닌 npm으로 설치하도록 수정합니다

before

RUN corepack enable

after

RUN npm install -g pnpm

Additional context

corepack 관련 이슈
nodejs/corepack#612

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[jimin9038]

corepack 고쳐지면 다시 사용할 수 있도록 주석 하나만 달아주실 수 있나요?

[jspark2000]

달았습니다!