Update READMEs to include instructions for mounting Conan cache volume (#77)

This PR mounts a Docker-managed volume where the Conan dependencies can be stored.

Author: bthomee

docker/debian/README.md

@@ -121,7 +121,9 @@ can do so with the following command:
 
 ```shell
 CODEBASE=<path to the rippled repository>
-docker run --user $(id -u):$(id -g) --rm -it -v ${CODEBASE}:/rippled ${CONTAINER_IMAGE}
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  ${CONTAINER_IMAGE}
 ```
 
 Note, the above command will assume the identity of the current user in the
@@ -140,6 +142,22 @@ If you see an error such as `bash: /root/.bashrc: Permission denied` and the
 prompt shows `I have no name!`, then exit the container and run it again without
 the `--user $(id -u):$(id -g)` option, or run it in rootless mode.
 
+#### Caching Conan dependencies
+
+You can further customize the `docker run` command by adding a volume mount for
+holding the Conan cache (the "p" directory), e.g.:
+
+```shell
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  --mount type=volume,source=conan,target=/root/.conan2/p \
+  ${CONTAINER_IMAGE}
+```
+
+This avoids the need to build the dependencies each time you run the image.
+
+#### Building the binary
+
 Once inside the container you can run the following commands to build `rippled`:
 
 ```shell

docker/rhel/README.md

@@ -78,7 +78,9 @@ can do so with the following command:
 
 ```shell
 CODEBASE=<path to the rippled repository>
-docker run --user $(id -u):$(id -g) --rm -it -v ${CODEBASE}:/rippled ${CONTAINER_IMAGE}
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  ${CONTAINER_IMAGE}
 ```
 
 Note, the above command will assume the identity of the current user in the
@@ -97,6 +99,22 @@ If you see an error such as `bash: /root/.bashrc: Permission denied` and the
 prompt shows `I have no name!`, then exit the container and run it again without
 the `--user $(id -u):$(id -g)` option, or run it in rootless mode.
 
+#### Caching Conan dependencies
+
+You can further customize the `docker run` command by adding a volume mount for
+holding the Conan cache (the "p" directory), e.g.:
+
+```shell
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  --mount type=volume,source=conan,target=/root/.conan2/p \
+  ${CONTAINER_IMAGE}
+```
+
+This avoids the need to build the dependencies each time you run the image.
+
+#### Building the binary
+
 Once inside the container you can run the following commands to build `rippled`:
 
 ```shell

docker/ubuntu/README.md

@@ -79,7 +79,9 @@ can do so with the following command:
 
 ```shell
 CODEBASE=<path to the rippled repository>
-docker run --user $(id -u):$(id -g) --rm -it -v ${CODEBASE}:/rippled ${CONTAINER_IMAGE}
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  ${CONTAINER_IMAGE}
 ```
 
 Note, the above command will assume the identity of the current user in the
@@ -98,6 +100,22 @@ If you see an error such as `bash: /root/.bashrc: Permission denied` and the
 prompt shows `I have no name!`, then exit the container and run it again without
 the `--user $(id -u):$(id -g)` option, or run it in rootless mode.
 
+#### Caching Conan dependencies
+
+You can further customize the `docker run` command by adding a volume mount for
+holding the Conan cache (the "p" directory), e.g.:
+
+```shell
+docker run --user $(id -u):$(id -g) --rm -it \
+  --mount type=bind,source=${CODEBASE},target=/rippled \
+  --mount type=volume,source=conan,target=/root/.conan2/p \
+  ${CONTAINER_IMAGE}
+```
+
+This avoids the need to build the dependencies each time you run the image.
+
+#### Building the binary
+
 Once inside the container you can run the following commands to build `rippled`:
 
 ```shell