A bug about dockerfile

## Instructions To Reproduce the 🐛 Bug:
1. Full runnable code or full changes you made:
```
docker build -t detectron2:v0 .
```
2. What exact command you run: docker build -t detectron2:v0 .
3. __Full logs__ or other relevant observations:
```
ERROR: Package 'detectron2' requires a different Python: 3.6.9 not in '>=3.7'
```
4. please simplify the steps as much as possible so they do not require additional resources to
run, such as a private dataset.

## Expected behavior:

it will build successful

## Environment:

Provide your environment information using the following command:
```
Failed to load OpenCL runtime
No CUDA runtime is found, using CUDA_HOME='/usr/local/cuda'
--------------------- --------------------------------------------------------------------------
sys.platform linux
Python 3.6.9 (default, Mar 15 2022, 13:55:28) [GCC 8.4.0]
numpy 1.13.3
detectron2 0.6 @/home/appuser/detectron2_repo/detectron2
detectron2._C not built correctly: No module named 'detectron2._C'
Compiler ($CXX) c++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
DETECTRON2_ENV_MODULE
PyTorch 1.10.0+cu111 @/home/appuser/.local/lib/python3.6/site-packages/torch
PyTorch debug build False
GPU available No: torch.cuda.is_available() == False
Pillow 8.4.0
torchvision 0.11.1+cu111 @/home/appuser/.local/lib/python3.6/site-packages/torchvision
fvcore 0.1.5
iopath 0.1.10
cv2 3.2.0
--------------------- --------------------------------------------------------------------------
PyTorch built with:
- GCC 7.3
- C++ Version: 201402
- Intel(R) Math Kernel Library Version 2020.0.0 Product Build 20191122 for Intel(R) 64 architecture applications
- Intel(R) MKL-DNN v2.2.3 (Git Hash 7336ca9f055cf1bfa13efb658fe15dc9b41f0740)
- OpenMP 201511 (a.k.a. OpenMP 4.5)
- LAPACK is enabled (usually provided by MKL)
- NNPACK is enabled
- CPU capability usage: AVX512
- Build settings: BLAS_INFO=mkl, BUILD_TYPE=Release, CUDA_VERSION=11.1, CUDNN_VERSION=8.0.5, CXX_COMPILER=/opt/rh/devtoolset-7/root/usr/bin/c++, CXX_FLAGS= -Wno-deprecated -fvisibility-inlines-hidden -DUSE_PTHREADPOOL -fopenmp -DNDEBUG -DUSE_KINETO -DUSE_FBGEMM -DUSE_QNNPACK -DUSE_PYTORCH_QNNPACK -DUSE_XNNPACK -DSYMBOLICATE_MOBILE_DEBUG_HANDLE -DEDGE_PROFILER_USE_KINETO -O2 -fPIC -Wno-narrowing -Wall -Wextra -Werror=return-type -Wno-missing-field-initializers -Wno-type-limits -Wno-array-bounds -Wno-unknown-pragmas -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wno-unused-function -Wno-unused-result -Wno-unused-local-typedefs -Wno-strict-overflow -Wno-strict-aliasing -Wno-error=deprecated-declarations -Wno-stringop-overflow -Wno-psabi -Wno-error=pedantic -Wno-error=redundant-decls -Wno-error=old-style-cast -fdiagnostics-color=always -faligned-new -Wno-unused-but-set-variable -Wno-maybe-uninitialized -fno-math-errno -fno-trapping-math -Werror=format -Wno-stringop-overflow, LAPACK_INFO=mkl, PERF_WITH_AVX=1, PERF_WITH_AVX2=1, PERF_WITH_AVX512=1, TORCH_VERSION=1.10.0, USE_CUDA=ON, USE_CUDNN=ON, USE_EXCEPTION_PTR=1, USE_GFLAGS=OFF, USE_GLOG=OFF, USE_MKL=ON, USE_MKLDNN=ON, USE_MPI=OFF, USE_NCCL=ON, USE_NNPACK=ON, USE_OPENMP=ON,
```

If your issue looks like an installation issue / environment issue,
please first try to solve it yourself with the instructions in
https://detectron2.readthedocs.io/tutorials/install.html#common-installation-issues

Start by reproducing the reported `docker build -t detectron2:v0 .` command and compare its Python 3.6.9 environment with the installation requirements in the linked common-installation-issues documentation. Trace which Docker build step installs detectron2 and verify the Python-version mismatch is resolved; done means the image builds successfully without the reported package error.

Written by the indexing model from the issue text.