Fix Docker image

The compliance test started failing in CI, while it continued to work
locally. There were two related issues.

1. The Cargo.lock file was not tracked by git, so CI would pull in the
   latest packages. One of these packages breaks the test. This change
   adds a Cargo.lock file with a set of working dependencies.
2. The Dockerfile uses cargo install rather than cargo build, which
   always ignores a Cargo.lock file. Adding the --locked argument causes
   it to use the Cargo.lock file.

Author: markgoddard

.gitignore

@@ -2,10 +2,6 @@
 # will have compiled files and executables
 /target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
@@ -19,4 +15,4 @@ Cargo.lock
 /target
 
 # Dev TLS certs
-.certs/
+.certs/