Dockerfile fails to build

[philpem]

Follow-on bug from #2.

When #2 is fixed, the Dockerfile still fails to build.

  creating build/temp.linux-x86_64-3.7
  creating build/temp.linux-x86_64-3.7/python
  creating build/temp.linux-x86_64-3.7/lib
  gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -DTHREAD_STACK_SIZE=0x100000 -fPIC -I./python -I./lib -I/usr/local/include/python3.7m -c ./python/ujson.c -o build/temp.linux-x86_64-3.7/./python/ujson.o -D_GNU_SOURCE
  unable to execute 'gcc': No such file or directory
  error: command 'gcc' failed with exit status 1
  ----------------------------------------
  ERROR: Failed building wheel for ujson
    ERROR: Command errored out with exit status 1:
     command: /usr/local/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-ek9e_eu5/ujson/setup.py'"'"'; __file__='"'"'/tmp/pip-install-ek9e_eu5/ujson/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-8y65xq9t/install-record.txt --single-version-externally-managed --compile
         cwd: /tmp/pip-install-ek9e_eu5/ujson/
    Complete output (12 lines):
    Warning: 'classifiers' should be a list, got type 'filter'
    running install
    running build
    running build_ext
    building 'ujson' extension
    creating build
    creating build/temp.linux-x86_64-3.7
    creating build/temp.linux-x86_64-3.7/python
    creating build/temp.linux-x86_64-3.7/lib
    gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -DTHREAD_STACK_SIZE=0x100000 -fPIC -I./python -I./lib -I/usr/local/include/python3.7m -c ./python/ujson.c -o build/temp.linux-x86_64-3.7/./python/ujson.o -D_GNU_SOURCE
    unable to execute 'gcc': No such file or directory
    error: command 'gcc' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /usr/local/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-ek9e_eu5/ujson/setup.py'"'"'; __file__='"'"'/tmp/pip-install-ek9e_eu5/ujson/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-8y65xq9t/install-record.txt --single-version-externally-managed --compile Check the logs for full command output.
The command '/bin/sh -c apk update &&     apk add --no-cache ca-certificates &&     pip install -r /opt/requirements.txt --no-use-pep517 --no-cache-dir -q --compile &&     rm -rf /var/cache/apk/*' returned a non-zero code: 1

To fix this, several lines need to be added to the Dockerfile's run line to install GCC, then remove it after building the Python libraries.
See the last comment on docker-library/python#318 .