Essentially, the key is D minor; though that is not to say that from an soloist's point of view, you should just stick to the notes that the key of D minor provides. For one, modal interchange is possible where variants of the minor scale ( in this example) built on the keynote of D can be juxtaposed: for example the harmonic, melodic versions of the natural minor scale...and the key of D Dorian (which has a B Natural rather than Bb).
In bar four, there may be a variant of D7 (with altered notes perhaps) which serves as a secondary dominant for the Gminor that follows. Depending on how the chord is altered will determine which scale is best to use with. The altered scale is the safest bet for all altered dominants (in this case D altered scale); though one can always just use the notes of the chord instead.
However, G minor is in the key of D minor, so the corresponding scale to improvise with would be G Dorian; but again, modal interchange allows other possibilities.
E Half diminished 7th followed by A7alt is a ii V7 to I in the key of D minor. Plenty of licks out there are designed to work with this - transpose to this key.
And then there is a II V I in the key of F (which is relative to Dminor); which is probably a little too transitory to be considered a modulation; before another II V I in Dminor re-establishes D as the key centre. An altered dominant at the end serves as a turnaround device to take us back to the start.