I have a strange graphics loading problem which I can't explain. I have this simple file docs/paper.tex

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\graphicspath{{./docs/}}
\includegraphics[scale=0.40]{teapot}
\end{document}


In the same directory as docs/paper.tex I have a file called docs/teapot.png. In my project root directory I call pdflatex:

$/usr/bin/pdflatex docs/paper.tex  It works fine. However, if I call latex: $ /usr/bin/latex docs/paper.tex
...
! LaTeX Error: File teapot' not found.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
...

l.656   \includegraphics[scale=0.40]{teapot}
`

Why does pdflatex and latex work differently here? And what is the right way to do it?