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

imagefiles/entrypoint.sh

@@ -50,8 +50,10 @@ if [[ -n $BUILDER_UID ]] && [[ -n $BUILDER_GID ]]; then
        gosu $BUILDER_UID:$BUILDER_GID /work/.dockcross
     fi
 
-    # Run the command as the specified user/group.
-    exec gosu $BUILDER_UID:$BUILDER_GID "$@"
+    # Run the command as the specified user.  The group will be BUILDER_GID,
+    # as pulled from /etc/passwd, but will have additional groups from
+    # /etc/group.  This is necessary for manylinux to install python packages.
+    exec gosu $BUILDER_UID "$@"
 else
     # Just run the command as root.
     exec "$@"