manylinux: add builder group to own python files

The manylinux images can take over a minute to start on spinning disks
(not SSDs) because pre_exec.sh is changing the ownership of all the
python install directories to be writable by the BUILDER_UID.  This is
detailed further in #853.

I changed the build steps for manylinux images to add a group called
`builder`, then change the python directories to be owned by the
`builder` group, and group writable.  I then changed pre_exec.sh to add
BUILDER_USER to the builder group prior to the gosu switch to become
BUILDER_USER.  The result is that BUILDER_USER is in the `builder`
group that can write to the python directories, allowing the user to
install python packages without permission denied errors.

There was one additional change to support this, and that was the
arguments to `gosu` in entrypoint.sh.  Previously the arguments were
`$BUILDER_UID:BUILDER_GID`, but providing the gid prevents `gosu` from
loading additional groups (such as `builder`) from /etc/group.  I
removed the GID argument to allow `gosu` to pick up additional groups
from /etc/group.

fixes #853

manylinux-common/install-python-packages.sh

@@ -4,3 +4,41 @@ for PIP in /opt/python/*/bin/pip; do
   $PIP install --disable-pip-version-check --upgrade pip
   $PIP install scikit-build==0.8.1
 done
+
+# pip is installing everything as root.  We need to make sure whatever uid/gid
+# is used when the image is run is able to update the python install.  We don't
+# want to limit which uid/gid can be used, so instead we're going to:
+#  - create a custom group
+#  - change the group of the python directories to the new group
+#  - change the permissions of the python directories to allow the group to write
+
+# This GID isn't used in the manylinux image, but just to be safe, we'll verify
+# it's still not in use during the image build.
+BUILDER_GID=200
+if grep "^${BUILDER_GID}:" /etc/group; then
+  echo "Error: builder gid $BUILDER_GID already exists!  Aborting"
+  exit 1
+fi
+
+# Add the builder group
+groupadd -g $BUILDER_GID builder
+
+# Update the permissions for all python directories so they're owned by the
+# builder group, and the builder group can write to them.
+for DIR in /opt/python/*/lib/python*/site-packages; do
+  chown -R :$BUILDER_GID $DIR
+  chmod g+w $DIR
+done
+for DIR in /opt/python/*/bin; do
+  chown -R :$BUILDER_GID $DIR
+  chmod g+w $DIR
+done
+for DIR in /opt/python/*; do
+  mkdir $DIR/man
+  chown -R :$BUILDER_GID $DIR/man
+  chmod g+w $DIR
+done
+for DIR in /opt/python/*/share; do
+  chown -R :$BUILDER_GID $DIR
+  chmod g+w $DIR
+done

manylinux-common/pre_exec.sh

@@ -1,15 +1,5 @@
 #!/usr/bin/env bash
 
-for DIR in /opt/python/*/lib/python*/site-packages; do
-  chown -R $BUILDER_UID:$BUILDER_GID $DIR
-done
-for DIR in /opt/python/*/bin; do
-  chown -R $BUILDER_UID:$BUILDER_GID $DIR
-done
-for DIR in /opt/python/*; do
-  mkdir $DIR/man
-  chown -R $BUILDER_UID:$BUILDER_GID $DIR/man
-done
-for DIR in /opt/python/*/share; do
-  chown -R $BUILDER_UID:$BUILDER_GID $DIR
-done
+# Add the BUILDER_USER to the builder group so `pip install` doesn't get access
+# denied errors.
+usermod -a -G builder $BUILDER_USER