Is the command bin/bash here because we're by default in / in this container? I suggest using /bin/bash just to be safe, and if we change WORKDIR later we won't need to tweak this.
Don't use output.o for the executable; *.o is commonly used for non-executable binary objects; there's no conflict here, but it's useful to follow the convention. You can use any name for the executable, e.g. executable, program, x, etc.
I suggest running the executable only if the compile step succeeds; i.e. cd fortran && gfortran Fortran.f90 -o output.o && ./output.o.
Quoting comment by @milancurcic from #2
A few questions and comments: