feat(cli): start the command in the mapped current directory

The default --bind overlay mounted the current directory at
/root/<basename>, but the command still started in /root: the
library's working-directory fallback only considers read-write
binds.  Pass the mapped target as the explicit working_dir so
relative paths keep meaning what they meant on the host.

Bind sources are canonicalized when the targets are built
instead of at mount time.  This fixes --bind . and other
basename-less relative paths, which cli_mount_target rejected,
and lets the --bind/--bind-rw dedup match relative and absolute
spellings of the same directory.

crates/ecr-cli/Cargo.toml

@@ -27,3 +27,7 @@ log = { version = "0.4", features = ["std"] }
 
 # Progress bar rendering of library progress events
 indicatif = "0.18"
+
+[dev-dependencies]
+# Temp directories for bind-target tests
+tempfile = "3"