Add gcc and other things to make it possible to install aicsimageio

Dockerfile

@@ -1,8 +1,13 @@
 FROM amazonlinux:2017.03
 
-RUN yum -y install git \
+# Need to set "ulimit -n" to a small value to stop yum from hanging:
+# https://bugzilla.redhat.com/show_bug.cgi?id=1715254#c1
+RUN ulimit -n 1024 && yum -y update && yum -y install \
+    git \
+    gcc \
     python36 \
     python36-pip \
+    python36-devel \
     zip \
     && yum clean all
 
@@ -11,3 +16,6 @@ RUN python3 -m pip install --upgrade pip \
     # but it's not in the amazonlinux image
     && python3 -m pip install boto3
 
+# Make it possible to build numpy:
+# https://github.com/numpy/numpy/issues/14147
+ENV CFLAGS=-std=c99