docker: include Alpine base tools, tini

Including tini fixes forwarding signals to MinedMap, allowing to
interrupt it using Ctrl-C. The base tools may be used to add a wrapper
script to configure MinedMap with environment variables.

As the Alpine base is included now, we can switch from the rust:alpine
image to alpine:latest, resulting in MinedMap to be linked dynamically.

.dockerignore

@@ -0,0 +1,4 @@
+*
+!/Cargo.*
+!/src
+!/crates