Reduce size of sratoolkit docker images

[gversmee]

Original size: 762Mb.
Change #1: Size decreased to 369Mb.

Other ways to decrease the size:

• use alpine
• remove wget

[mikaeldusenne]

274.1MB using Alpine, cf #15

[gversmee]

Great! Although with further thoughts, I wouldn't push any modification at this point given that we cannot test them.