Fix Dockerfile conda install error for some shells (pytorch#92702)

The issue was first solved in [pytorch/pull/91371] for CI/CD, but the main Dockerfile in the repo root still has this issue for people trying to test build custom image manually.
Without it the build fails at installing miniconda
```
#14 3.802 Preparing transaction: ...working... done
#14 4.087 Executing transaction: ...working... done
#14 5.713 /root/miniconda.sh: 438: /root/miniconda.sh: [[: not found
#14 5.713
#14 5.713 Installing * environment...
#14 5.713
#14 5.714 /root/miniconda.sh: 444: /root/miniconda.sh: [[: not found
#14 6.050
#14 6.050 CondaFileIOError: '/opt/conda/pkgs/envs/*/env.txt'. [Errno 2] No such
file or directory: '/opt/conda/pkgs/envs/*/env.txt'
#14 6.050
```

With the modification, locally tested build successfully with `make -f ./docker.Makefile` as instructed in the README

Pull Request resolved: pytorch#92702
Approved by: https://github.com/seemethere, https://github.com/malfet

Dockerfile

@@ -36,8 +36,9 @@ RUN case ${TARGETPLATFORM} in \
  esac && \
  curl -fsSL -v -o ~/miniconda.sh -O "https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-${MINICONDA_ARCH}.sh"
 COPY requirements.txt .
+# Manually invoke bash on miniconda script per https://github.com/conda/conda/issues/10431
 RUN chmod +x ~/miniconda.sh && \
- ~/miniconda.sh -b -p /opt/conda && \
+ bash ~/miniconda.sh -b -p /opt/conda && \
  rm ~/miniconda.sh && \
  /opt/conda/bin/conda install -y python=${PYTHON_VERSION} cmake conda-build pyyaml numpy ipython && \
  /opt/conda/bin/python -mpip install -r requirements.txt && \